Concrete Stamp Made from Urethane Rubber

February 14, 2018

Urethane rubber has many uses because of its strength, durability, and flexibility. It is often used to pot or encapsulate underwater electronics, make shoe insoles, make molds for concrete bricks, blocks, and statues. It also makes an excellent stamp for concrete, where it is pressed into fresh, wet concrete to create a pattern. In this post, I’ll show you how to make a urethane rubber concrete stamp.

Materials needed:

  • Tiles, or something that has a pattern that you want to stamp into concrete
  • AeroMarine Products 75A Urethane Rubber with Mold Release
  • AeroMarine Products AM128 Silicone (not necessary for all projects)
  • Spray paint or spray sealeant
  • Plastic mixing cups and utensils
  • Hot glue gun
  • Cardboard
  • X-acto knife

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • For this project, I found interestingly shaped tiles that I want to use to make a concrete stamp. Since I’m not doing a driveway or other large scale concrete stamping application, I won’t make a large 2ft x 2ft original with tiles thin-set and grouted into place. Instead, I’m making a smaller concrete stamp as a decorative accent.  I will use silicone to reverse the tiles, so that when I make the concrete stamp, I’ll be pressing the shape of the tile into the concrete, rather than pressing the space between the tiles into the concrete. When using a concrete rubber stamp to mimic a tile or brick pattern over a large area, you will do the opposite, so that the tile/brick pattern looks like the real thing.
  • First, I cut out the cardboard and glued it together with hot glue to make a mold box.
  • Next, I glued my tiles down and mixed up some AeroMarine Products AM128 Pourable silicone. I picked a corner and poured the silicone slowly, letting it flow around the tiles. After 24 hours, the silicone cured. I removed it from the mold box. Again, if you are making a stamp to mimic brick/tile work for a large area, make the mold box around your tile/brick work, but don’t pour silicone into it. You would apply mold release to the tiles and pour the 75A Urethane into instead. I go in to more detail on mixing and pouring the urethane in the next few steps below.


  • For the next step, I need to construct another mold box around the silicone. However, this is the box into which I will be pouring the 75A Urethane Rubber.  Urethane is moisture sensitive before it is cured. Things like paper, wood and cardboard can retain moisture from the atmosphere and cause bubbles in the urethane. So, I am going to cut out my cardboard for the mold box and then seal it with a clear acrylic spray. You can also just paint it using any old can of spray paint you have around.

  • Once the sealant is dry (read the directions on the can), I used my hot glue gun to build the box around the silicone piece. This is easier than building the box and then trying to put the silicone piece in it.

  • Before I mix and pour the 75A Urethane Rubber, I need to apply some mold release to the sealed cardboard. This 75A Urethane Rubber kit comes with a silicone based mold release paste that I am going to apply with a clean rag.

  • Now I’m ready to mix my 75A Urethane Rubber. The mix ratio on this product is 1:1 by volume. I mixed a pint of part “A” and a pint of part “B” vigorously by hand for about 60 seconds and then poured it into the mold box. This product has a short pot life, at about 3 minutes at  70 F, so mix quickly and thoroughly, taking care to scrape the sides and bottom of the mixing container.  The full cure takes 12 hours, so I’ll come back to de-mold tomorrow.

  • Fully cured now, my concrete stamp is ready to go!


  • Because the urethane rubber is pretty stiff, I poured it thinner than I did the silicone. The AM128 Silicone has a Shore A hardness of 28, which is a little firmer than a rubber band. The 75A Urethane Rubber has a Shore A hardness of 75, which is about the firmness of a car tire. So I always want my silicone to be about 1 inch thick, but the urethane rubber can be 1/4 to 1/2 inch thick.
  • The finished stamp can now be pressed into concrete!

If you have any questions about this project or your own projects, give us a call at 877-342-8860 or drop us an email at

Polyester Resin Vs. Epoxy Resin

January 24, 2018

 Epoxy resin makes a strong and durable tabletop.

We frequently get asked about the difference between epoxy resin and polyester resin. Often, people say “resin” without specifying which resin. It all depends on what your project is as to whether you would choose epoxy resin or polyester resin. Here’s a quick breakdown on the differences between epoxy resin and polyester resin.

Epoxy Resin Characteristics:

  • Epoxy is extremely strong and has great flexural strength. Our 300/21 and 400/21 epoxy resins have a flexural strength of 17,500 psi. Our 300/11 and 4/11 epoxy resins have a flexural strength of 12,800 psi.
  • Epoxy has a great shelf life. While we guarantee our epoxy resins for 6 months, you can reasonably expect the epoxy to last at least a few years properly  stored on the shelf.
  • You cannot vary the cure time of epoxy resin. Epoxy resin’s cure time is determined by the hardener and the temperature. Our 300 resin with 21 hardener has a cure time of 24 hours at 70 F.  Adding more hardener will not make it cure more quickly! Too much hardener will actually make the cured epoxy rubbery. You can decrease the cure time by raising the ambient temperature around the epoxy resin. For every 10 F the temperature is raised above 70F, you can cut the cure time by 10%. However, don’t raise the temperature over 100 F.
  • A gallon of epoxy generally costs slightly more than polyester resin.
  • Epoxies like ours that are 100% solids are low to no VOC. They have a hardly any odor.  You don’t need a filter mask when working with epoxy. Epoxy is also non-flammable.
  • Epoxy cures fully when applied as a thin film, making it ideal for adhesive applications.
  • Epoxy does not shrink when cured.
  • Epoxy is very resistant to wear, cracking, peeling, corrosion and damage from chemical and environmental degradation.
  • Once cured, epoxy is moisture resistant.
  • Epoxy is not, on its own, UV resistant. Some epoxies have a UV resistant additive added to them that works moderately well. However, the best way to ensure that your epoxy doesn’t yellow or breakdown from UV rays is to top coat it with clear UV resistant urethane. Then, you will have the strength and durability of the epoxy and the UV protection of the urethane.
  • Epoxy generally has a bonding strength of up to 2,000 psi.
  • Epoxy will not bond to polyethylene, polypropylene, EPDM, anodized aluminum, Teflon or Tedlar.

Polyester Resin Characteristics:

  • Polyester resin is brittle and prone to micro-cracking.
  • The shelf life of polyester resin is about 6 months – 1 year.
  • You can vary the cure time of polyester resin by adding more or less of MEKP catalyst.
  • Polyester resin generally costs slightly less than epoxy resin.
  • Polyester off-gases VOCs and has strong, flammable fumes.
  • Polyester resin does not cure well as a thin film, so it doesn’t work well as an adhesive.
  • The larger the pour, the more polyester resin shrinks when cured.
  • Polyester resin is UV resistant and does not need to be top coated to prevent yellowing or degradation from sunlight.
  • Once cured, polyester resin is water permeable, meaning water can pass through it eventually.
  • The bonding strength of polyester resin is generally less than 500 psi.
  • Polyester resin will not bond to epoxy resin.

With its strength, glossy finish and durability, epoxy resin is ideal for things like bar tops, table tops, and counter tops,


Building and repairing wooden and fiberglass boats,

Making fiberglass and carbon fiber parts,


Coating wood and concrete,

Casting small objects,

As well as flooring, potting electronics, bonding wood, metal, concrete and many types of plastic, sealing and patching wood that has dry rot, pebble paving, and many more applications where thorough bonding, strength, glass-like finish and durability are required.

If you have any questions about what product would be best for your project, send us an email at or give us a call, toll-free, Monday -Friday 9am- 4pm Pacific time at 877-342-8860. We are always happy to answer any questions you may have!


Epoxy and Wood Star Holiday Decoration

January 8, 2018

Epoxy can help make wooden decorations last a long time. For this project, I will be painting and decorating a large plain wooden star that I found at a local craft store and then sealing it with epoxy. This application process can be used with any size and shape wood decoration. Make sure to use a water based stain/paint. Oil based paint/stain is quite difficult for the epoxy to bond to it. It takes a lot of thin coats and sanding to get the epoxy to look nice over oil based stain/paint. It can be done but it just takes a lot of work and that is a project for another blog post.

Materials needed:

  • Wood star (or other shape)
  • AeroMarine Products 300/21 Epoxy Resin and Hardener
  • Water based metallic wood stain/paint
  • Glitter
  • Blue painter’s tape
  • Plastic measuring/mixing containers and utensils
  • Inexpensive brushes (I used 1 inch chip brushes)
  • Gloves
  • Acetone and clean oil free rag

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • This star had a loop of twine at the top of it, so I taped that down and out of the way first. Then, I applied a water based metallic stain/paint according to the product directions. I used a 1 inch chip brush and applied 2 stain coats, waiting 12 hours between coats. After the last coat, I waited 72 hours before applying the first coat of epoxy.

  • Before I applied my first coat of epoxy on the whole star, I did a little test area on the side of the piece. I did this because it was my first time using this particular stain/paint. I didn’t think there would be any issue because it is a water based material intended to go over wood, but better safe than sorry. The test spot turned out fine, so I proceeded with the full application.

  • The 300/21 Epoxy is mixed 2:1 by volume. I measured, mixed in a clean container, then after mixing for a couple of minutes, poured it into a new, clean container and mixed for a few more minutes. I did this to make sure the epoxy was completely mixed and there wouldn’t be an unmixed material clinging to the sides of the mixing container. Any unmixed resin or hardener will make it difficult for the the epoxy to properly cure. I mixed up a total of 3 ounces, which is a very small batch.
  • Next, I used a 1 inch chip brush to apply a nice thin coat all over the star, brushing the sides as well.
  • I let each coat of epoxy cure for at least 24 hours before applying the next coat. If I had mixed a larger amount of epoxy (at least 6 ounces), if the weather was warmer or if I had a heater running in my work space, I would have only waited 12 hours between coats. Smaller batches and colder temperatures cause the epoxy to cure more slowly which is why I gave it each coat more time to cure. This epoxy does not need to be sanded between coats, unless it has been longer than 2 weeks since the previous application.

  • Once I had 3 brush coats of epoxy applied and cured, I taped off the edges of the star with painter’s tape. I want to apply glitter just to the center of the star and have a nice border of just plain gold.

  • Once the edges were taped up, I mixed up and applied another coat of epoxy just to the middle of the star. Then, I sprinkled glitter on to the wet epoxy. I let it cure for 24 hours and the applied another coat of epoxy.

  • After the epoxy cured, I removed the tape. I used some acetone on a clean, oil free rag to wipe the entire surface of the star, focusing on the areas that had been taped. You really want to make sure the cloth/rag you are using does not have any oil residue on it. Oil contamination will make the final coat of epoxy look uneven.
  • After the acetone has evaporated (just a few minutes), I mixed and applied the final coat of epoxy. The last coat will even out the difference between the glitter area and the border.


You can see the sharply delineated edge between the glitter and border and how the final coat really smoothed the whole piece out. This whole project took about week, applying a coat of epoxy a day. You can check out our epoxy page here,

As always, if you have any questions about this project or your own projects, send us an email at


Silicone Glove Mold and Fiberglass Mothermold

November 14, 2017

With larger pieces, it is more cost effective to make a glove mold rather than a large block mold. A glove mold is also less cumbersome than a large block mold. Glove molds do need a supportive structure since they are thinner than a block mold, so I will make an epoxy and fiberglass mothermold to support the finished glove mold. Additionally, the piece I picked for this project has a lot of detail so I will be using a silicone diluent to pick up all the detail and decrease the risk of air bubbles.

Materials needed:

  • Original part
  • AeroMarine Products 128 Silicone with Brushable Catalyst
  • AeroMarine Products Silicone Diluent
  • AeroMarine Products 300/21 Epoxy Resin
  • AeroMarine Products 6oz Plain Weave Fiberglass Cloth
  • Scale
  • Measuring and mixing containers and utensils
  • Piece of cardboard large enough for original to sit on
  • Inexpensive brushes (I used 1 inch chip brushes)
  • Gloves
  • Scissors for trimming fiberglass

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • To start, I set my original on the cardboard, right in the middle. The mold needs to have a lip at the base. The cardboard is something that the silicone can be applied to and will peel off easily when I’m done.


  • Because of the detail on the original, I am using silicone diluent to thin the rather viscous brushable silicone. The diluent is added to the white silicone base (Part A) up to 10% by weight. For the first coat, I weighed out 150 grams of silicone Part A and added 15 grams of the silicone diluent and mixed well. The total weight of my silicone is now 165 grams and the catalyst needs to be added at 10% of the total weight. I rounded 16.5 grams up to 17 grams because my scale doesn’t do half grams. This silicone isn’t finicky or exacting, so you can round up or down. It will turn out fine either way.
  • As I applied the silicone using a 1 inch chip brush, I really focused on getting into all the nooks and crannies of the piece. This silicone has a 45 minute pot life which allows me to take my time and makes sure there are no air bubbles in my first coat of silicone. Any air bubbles in the first coat will be recreated as lumps in every casting. So while I try to avoid air bubbles in every subsequent coat, it is extremely important to not have any in the first coat.

  • I let the silicone cure for 24 hours. For the next two coats, I mixed 150 grams of Silicone Part A, 8 grams of silicone diluent (5%), and 16 grams of catalyst. The 2nd and 3rd coats can go on a little thicker to help build the mold. I also focused on making a “lip” all the way around the piece, extended about an inch out. I always let each coat cure for 24 hours before applying the next coat.


  • For the 4th and final coat, I went back to adding the silicone diluent at 10% because I want this coat to flow more into the remaining detail and be mostly smooth on top. This makes it easier to do the epoxy and fiberglass mothermold. I let the silicone cure for 24 hours before moving on to the next step.

  • Now that I have a nice smooth glove mold, it’s time to make an epoxy and fiberglass mothermold to support it. First, I peel the whole thing off the cardboard, taking care not to peel the silicone off the piece. I set it back down on the plastic covering on my work station. The epoxy peels easily off the plastic, but would stick to the cardboard.


  • I’m using our 6 oz plain weave fiberglass. Since this piece is mostly flat and not too large, I’m only going to do one layer of resin and fiberglass mothermold. If the piece was larger or more vertical, I would do several layers to give it more support.
  • I trimmed the fiberglass to fit comfortably around the mold and then set it in place. I pressed the fiberglass into the contours of the mold. This was fairly simple because I filled in most of the detail with the last coat of silicone.


  • Next I mixed 4 oz of AeroMarine Products 300 Epoxy Resin with 2 oz of AeroMarine Products 21 Epoxy Hardener. I stirred it vigorously, taking care to scrape the sides and bottom of the mixing container, for several minutes. Then, I dribbled the epoxy around the edges of the fiberglass to hold the edges down. I used my chip brush to apply more epoxy to fiberglass, saturating it completely.

  • If I needed to do more layers, I would let this set up for at least 12 hours and then follow the same procedure, this time laying the fiberglass in the opposite direction for added strength. Since I’m only doing the one layer, I just let it cure for the full 24 hours before coming back to remove the fiberglass mothermold and de-mold the piece.
  • The raw edges of fiberglass and epoxy were very sharp, so I used heavy duty scissors to trim and shape teh fiberglass mothermold.


  • Once the sharp edges were trimmed away, I carefully pulled the mold (with the piece still inside) away from the fiberglass mothermold. Then, I carefully removed the silicone mold from the piece.

  • If I was going to cast, I would then sit the mold back into the mothermold for the casting.

Done! This was just a simple glove mold with epoxy and fiberglass mothermold. The same method can be applied to larger and/or more complex projects. As every project is different, give us a call, toll-free, at 877-342-8860 or drop us an email at if you have questions about your own project!

Latex Storage and Winter Weather

October 30, 2017

Latex is a great mold making material that makes durable, long lasting molds for concrete, plaster, hydrocal and other non-exothermic casting materials. Finished latex molds have a long library life when properly stored as well. Howeve,r winter is coming and that can mean issues with shipping and storing uncured latex.

Our AeroMarine Products Mold Making Latex is a creamy white color with a smooth texture when uncured. It cures to a light beige color. Here is what our uncured, properly stored latex looks like:

To keep your latex viable, it needs to be stored indoors at a temperature between 60F and 72F. If the latex gets below 60F, it can freeze which ruins it. Frozen latex becomes clumpy and cottage cheese like and unusable. Once the latex has frozen, it cannot be revived and used again. Here is what latex that has been frozen looks like:

  Before and after stirring 

Do not store your latex in a garage, basement, shed or any other unheated building during winter. Proper latex storage should be indoors in a heated building in a locked cabinet or closet, away from any pets or small children. When kept at the appropriate temperature, your latex should last for six months.

On the shipping side of things, we put big “PROTECT FROM FREEZING” stickers on all the latex we ship out during the months of October-April. When shipping the latex to the East Coast and other colder climates, we also generally ship 3 day shipping instead of ground during the colder months. This reduces the time the latex may spend in cold weather in transit. We ask UPS to require a signature on latex shipped during the winter to ensure that no packages containing latex are left on porches during cold weather. This method works well. We generally don’t have an issues with our customers receiving frozen latex. However, if you receive frozen latex, you can email us a picture of the latex ASAP and we will send you a replacement.

Remember, proper latex storage is indoors, between 60F and 72F.  Latex should be used within 6 months from date of delivery. Remember to check the Technical Data Sheets for any other materials that you may be storing over the winter. Many have special temperature and storage requirements. You can also read these blog posts for more information:

Curing Issues- Trouble Shooting

If you have any other questions, drop us an email at or give us a call Monday-Friday 9am-4pm Pacific time at 877-342-8860.

Penny Tabletop with Epoxy

September 7, 2017

For this project, we are taking an old side table and refinishing it with pennies and epoxy. Penny epoxy counter tops, table tops, and floors are a popular project that we get asked about frequently. This post will walk you through doing a penny epoxy table top over wood. If you have any questions about doing a floor or counter top over concrete, you can always send us an email at or give us a call toll-free at 877-342-8860.

Materials needed:

  • Wooden table
  • AeroMarine Products 300/21 Epoxy Resin and Hardener
  • Water based wood stain
  • Acetone
  • Sand paper (I used 60 grit)
  • Lots of pennies (I used about $4.25 for a foot square tabletop)
  • Paper barrier mask and gloves
  • Plastic drop cloth or similar
  • Inexpensive brushes (I used 1 inch chip brushes)
  • Mixing and measuring containers and utensils
  • Blue painter’s tape
  • Paint (for the legs of the table)

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • First, I needed to sand the old paint off the top of the table. Our epoxy sticks to most paint but I want a lighter background for the pennies. I also want the epoxy to penetrate and bond to the wood, not the paint, so that there is no worry about de-lamination over time. I used 60 grit sandpaper to sand off all the old paint.


  • When I was done sanding, I wiped the table off with a dry rag to clean off all the wood dust. Then, I applied the first coat of my water based wood stain. It is a lot of work to get epoxy to stick to oil based stain, whereas epoxy goes over the water based stain easily. Follow the application directions for the water based stain that you choose. The stain I was working with said to brush on a coat and then wipe off any excess immediately and wait two hours between coats. I did three coats following those directions and then let stain set over the weekend.

  • While I was waiting for the stain to dry, I laid out my pennies. I chose to arrange them from brightest to darkest for a gradient look. This also allowed me to check the pennies and make sure none of them had large amounts of dirt on them.

  • On Monday morning, I brushed on a thin coat of AeroMarine Products 300/21 Epoxy using a 1 inch chip brush. The 300/21 is mixed 2:1, 300 resin to 21 hardener. I mixed up 3 ounces for the brush coat, which was a bit more than I needed but it kept the mix ratio easy to measure. Mixing up less than 3 ounces can make getting the mix ratio right rather tricky. Very small batches will also take longer than the standard 24 hour cure time. On Tuesday morning, I did the same process. The reason that we do a couple of brush coats first is to seal the wood and cut down on air bubbles when we get to the pour coats. On Wednesday morning, I brushed a coat of epoxy on and then set my pennies on the epoxy immediately after brushing on the epoxy. I like using the epoxy to glue down the pennies rather than using a super glue or something similar because it is one less product to purchase. Also, there is less space for air to get trapped under the pennies and cause air bubbles later.
  • Thursday morning, I checked to make sure the epoxy had set up and the pennies were glued into place. Everything looked good, so I taped up the edges of the table with painter’s tape to prep for the first pour coat.

  • It takes a couple layers of tape to make sure the bottom is sealed completely and the edge of the tape rises above the pennies.
  • I mixed up 6 ounces of epoxy for the first pour coat. I poured the epoxy on the pennies and then used a brush to spread the epoxy out. The aim of this first pour coat was just to fill in the spaces between the pennies. I like to do multiple pour coats because the thinner the pour, the easier it is for air bubbles to rise up to the surface and pop. Additionally, if there are any air bubble once the epoxy cures, it is easy to sand them out before doing the next pour coat.
  • After 24 hours, I pulled the tape off and checked for bubbles. There were a few bubbles on the surface and along the edge where the tape was. I sanded these bubble out with 60 grit sandpaper and wiped the whole table top with acetone on a clean rag. Then I taped the edges up again. Then, I mixed another 6 ounces of epoxy and poured another coat, this time covering up the pennies completely.

 Surface bubbles

Surface bubbles sanded out

  • I decided to do one more pour coat to give the epoxy more thickness and depth. After the third pour coat, I pulled the tape off and sanded down the edges where the epoxy met  the tape. The epoxy wicks up where it meets the tape so it is important to sand it flat. That also happens to be where bubbles like to collect, so sanding serves two purposes.

 You can see some bubbles and the wicking effect along the edge

  • The next step is to do a thin pour coat that flows over the sides of the table. Once the tabletop was all sanded, I spread out a large trash bag underneath the table. This table also has some slats that connect the legs near the bottom that I covered with tape to protect them from epoxy dripping off the edges of the table. I mixed up another 6 ounces of epoxy and poured it on the table and used a chip brush to spread it out a bit. The epoxy will run over the sides, so use the brush to brush the epoxy along the edge. Once the epoxy starts to gel, about 20-30 minutes, I used the back of a plastic knife to scrape the drips off.

 Drips hanging off the underside of the table

 Plastic knife used to scrape those drips off

  • If there are any bubbles once the epoxy has cured, you can sand them out and do a thin brush coat. Fortunately, I didn’t get any bubbles in the final pour! Now, I’m ready to paint the table legs.

                           Penny Epoxy Tabletop

  • Finally, I taped up the edges of the table and applied two coats of forest green spray paint. Follow the directions for the paint you choose in regards to surface prep and dry time.

 Taped edges, ready to paint

 All done, just drying


I got little green paint on the epoxy of one corner when I turned the table to paint the other side of the legs. It easily wiped away with some acetone.

This project was done on a small wooden table, but penny epoxy flooring, counter tops and larger tables are also common. Hopefully, this post gives you a good idea of the process. If you have any questions about the products used or general application questions, check out our website, or send us an email at or give us a call at 877-342-8860.


Silicone Mold to Modify a Fragile or Broken Original Part (Post 2 of 2)

July 12, 2017

This is a continuation of our last post about using silicone molds to modify a broken or fragile original part. In this post, we will go over sculpting in clay on the new piece and making a new mold. You can read the first part of this blog post here:

Materials needed:

  • AeroMarine Products AM128 Pourable Silicone
  • AeroMarine Products White Urethane Casting Resin
  • Urethane colorant
  • Mold release
  • Original part
  • Modeling clay
  • Mold box
  • Chip brush
  • Scale
  • Mixing containers and utensils
  • Gloves

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • Now that I have a original, I can use clay to sculpt directly on to it. I’m using non drying, sulfur free modeling clay. Because I’m using a non drying modeling clay, I want to make sure that what I’m sculpting will be able to be pulled from a one piece or clam shell mold. If I want to sculpt something more intricate, it would require a two part or multi piece mold. For a two part mold or multi piece mold, I would use a clay that air dries for sculpting and then use the modeling clay when making the mold. You can watch our video about making two part molds here:
  • I decided I wanted my part to have two versions, unicorn horn and monster horns. I cast two parts in urethane casting resin from the silicone mold and sculpted the unicorn horn on one and the monster horns on the other. If you want to save on materials, you can certainly use one cast, make the mold, remove the clay and then sculpt on that part again.


  • I reused the same plastic containers for these new molds, so I mixed up the same amount of silicone (900 grams part A, 90 grams of part B catalyst) as I used for the first mold. Because of the detail of these pieces, I again used a chip brush to brush a thin coat of silicone on to the pieces before slowly pouring the rest of the silicone into a corner of the mold box.


  • About 16- 24 hours later when the silicone cured, I removed the original part.
  • I checked to make sure the humidity was under 50%, sprayed a little mold release into the molds and got ready to mix up my urethane casting resin.
  • The AeroMarine Products White Casting Resin is mixed 1:1 by volume. Urethane colorant is always added to the part B and well mixed before combining the two parts. Make sure to mix vigorously for about a minute with a plastic utensil and then pour slowly into the lowest part of the mold. AeroMarine Products Urethane Casting Resin is ready to demold in 30 minutes. Because of the shape of the part with the horns, I found it is best to demold right at the 30 minute mark when the casting resin is still warm and a bit pliable. It makes it much easier to remove the part from the mold. Once the part is removed from the mold, I set it aside to allow it to continue hardening and cooling off.

  • The cast on the right was colored with green urethane colorant. The cast on the left was painted with orange glow-in-the-dark paint with the horns colored with permanent felt tip marker.

The final results! All of these were achieved by a combination of colored resin, paint, and felt tip markers. As always if you have any questions, send us an email at

Silicone Mold to Modify a Fragile or Broken Original Part (Post 1 of 2)

June 8, 2017

Sometimes, I have an original part that is either broken or that I want to modify in different ways. In this 2 part blog post, I am going to make several slightly different variations of this part. However, the original is a bit fragile. I don’t want to damage it by sculpting clay on it and then making a mold from it several times over. So, I’m going to make one mold, cast a piece in urethane resin, and use the urethane resin part to sculpt on and make new molds from it. If the part was broken, I would mold and cast the original and then use clay to fill in the broken areas of the original part.

Materials needed:

  • AeroMarine Products AM128 Pourable Silicone
  • AeroMarine Products White Urethane Casting Resin
  • Urethane colorant
  • Mold release
  • Original part
  • Modeling clay
  • Mold box
  • Chip brush
  • Scale
  • Mixing containers and utensils
  • Gloves

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • If your original is unpainted clay, old concrete or plaster, or another similarly porous material, seal it before making the mold. You can do this by painting the part, or using a sealant type material. For my clay original part, I used an clear aerosol urethane sealant.

  • Anchor the original to the bottom of the mold  box with a small amount of modeling clay. For this mold box, I used a plastic food storage box because it happened to be just about the perfect size. You can always make a custom mold box using cardboard and a hot glue as well. This blog post covers how to make a mold box,


  • I weighed out about 900 grams of the silicone Part A base and 90 grams of the Part B catalyst and mixed well until it was a nice uniform pale purple color. The mixing took about five minutes.
  • Since this piece has a lot of fine detail, I used an inexpensive chip brush to brush the silicone into the detail of the piece before pouring the rest of the silicone. This AeroMarine Products AM128 Pourable Silicone has a pot life of 45 minutes, so there was plenty of time to brush some on before slowly pouring the rest into the mold box.

  • 20 hours later, I removed the mold from the mold box and the original part from the mold. It took a little wiggling, because of the undercut of the beard, but it all came out in one piece.

  • Then, I ran into a problem. I will cast the piece in a white urethane. The weather here in San Diego was very uncooperative for about a week. It was overcast and muggy (June Gloom, we call it). The humidity didn’t get below 50%, which is the threshold for working with urethane. Urethane is very moisture sensitive before it is cured.  Mixing and pouring a urethane on a humid or rainy day will result a lot of bubbles in the piece. So, I put this project on hold until the weather cleared up.
  • When the weather finally cleared, I washed the mold with warm water and dish soap, patted it dry and then let it air dry as well. I did this because it had been sitting a dusty warehouse for over a week. Once it was completely dry, I sprayed some Urethane Mold Release into the mold and let it sit for a few minutes. This lets the propellants evaporate and leave behind just the mold release.

  • I mixed my AeroMarine Products Urethane Casting Resin 1:1 by volume, stirring vigorously for about a minute and then poured slowly into the mold. Not super slow because the pot life for the Urethane Casting Resin is only about 3-5 minutes, but slowly enough to let it flow through mold without causing air bubbles.


  • It’s hard to tell from the second picture, but all the little bubbles from pouring the material have popped and the AeroMarine Products Urethane Casting Resin is smooth and bubble free.
  • After thirty minutes, I popped the cast out of the mold. Because of the mold release, it came out very easily.

  • An excellent, sturdy replica of my fragile, original part.

In the next blog post, I’ll go over sculpting, re-molding, and using the urethane casting resin and colorant to create new pieces.

Roto Casting Resin in a Two Part Silicone Mold

May 12, 2017

Rotational or roto casting means rotating your mold continually while the resin sets up so that youget a hollow casting. Two of the most common reasons to cast this way are to keep the weight of the piece down and to save money on casting materials. For this post, I’m doing the roto casting by hand. The internet is full of plans and how-to videos for making rotational casting frames, either motorized or hand turned method. If you think roto casting is something you will be doing for a lot of your pieces, it’s definitely worth a Google search to see what’s out there.

Materials needed:

  • Two part silicone mold (refer back to this post if you haven’t made a silicone mold yet )
  • AeroMarine Products Urethane Casting Resin
  • Non-sulfur modeling clay
  • Tape (I prefer blue painter’s tape, but masking tape and other varieties are fine too)
  • Plastic mixing containers and utensils
  • Gloves
  • Urethane colorant (optional)

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • To start, I took my two part mold and taped it shut, leaving the vent and sprue (opening for pouring the product into) unobstructed.


  • I blocked the vent with some clay and then applied more tape to hold the clay in place.

  • Next, I got ready to mix the Urethane Casting Resin. This product is mixed 1:1 by volume. This mold usually takes about 14 ounces to fill, so I figured out that I would use about 6 ounces to do the roto casting. I measured out 3 ounces of Part A and 3 ounces of Part B. I mixed some blue urethane colorant into the Part B and then mixed both parts together. I carefully poured the resin into the sprue, wiped away the little bit that got on the outside of the mold, plugged the sprue with clay and taped it shut.

 Blue colorant mixed into Part B 

  • I marked the bottom of the mold with a star so I know which side to set the mold down on when I’m done rotating the mold. I want to set the mold bottom down so that any excess resin settles at the bottom of the casting. Otherwise, the casting will be heavier on whatever side the excess resin settles, making it prone to falling over.
  • Once everything was taped up, I picked up the mold and rotated it by hand for 10-15 minutes. I turn it every which way, so that the resin will completely coat the inside of the mold.
  • After 10-15 minutesof rotating, I set the mold star side (bottom side) down and let it sit for the remaining 15 minutes of it’s cure time.
  • After 15 minutes, I cut the tape off, opened the mold and popped out my casting. I trimmed a small amount of flash from the parting line and sprue.


This roto casting turned out nicely. As always, if you have any questions, about this or other projects, please send us an email at

Silicone Additives- Accelerator, Thinner, Thickener

May 1, 2017

Our tin catalyzed silicone, AeroMarine AM 128 Silicone Rubber, can be used with silicone additives that either decrease or increase the viscosity or speed up the cure time. Each of these silicone additives is added at different times in the mixing process and in different quantities to affect the silicone. These additives are the proper way to control viscosity and speed up cure time. Please do not mix silicone with both the pourable (purple) and brushable (pink) catalyst. Please select one catalyst and use the thinning or thickening additive to control the viscosity.  All of the silicone additives discussed here are for use with tin catalyzed silicone where the silicone and catalyst are separate until mixing. They are not to be used with AM 125 Silicone Rubber, a 1:1 silicone where one side contains the catalyst or our platinum catalyzed silicones, like our Food Grade Silicone or our Silicone Putty.

  • Silicone Accelerator:

This silicone additive allows you to speed up the cure process from the usual 16 to 24 hours. It is added by weight to the catalyst, NOT to the white silicone base. You must use a digital gram scale for accurate weight measurement. The silicone accelerator should only be added to the catalyst at the time you are using it. The accelerator loses its potency when added to the catalyst and left to sit. It is important to remember that speeding up the cure time also shortens the pot life. So, the silicone needs to be mixed and poured in or brushed on quickly when using the accelerator. Here are some guideline weight percentages with their pot lives and cure times:

.2% by weight of accelerator to catalyst gives a 20 minute pot life and a 3 hour cure time

.6% by weight of accelerator to catalyst gives a 5 minute pot life and a 2 hour cure time

1.0 by weight of accelerator to catalyst gives a 3 minute pot life and a 1 hour cure time

You don’t want to add more than 1.0% to the mix because it can cause the finished mold to be brittle.

  • Silicone Thinner/Diluent:

The silicone thinner is added to the silicone base (part A) to decrease the viscosity of the silicone. This allows the silicone to flow more easily over intricate parts and also de-air faster. It can also be used with the brushable catalyst to achieve a thinner first coat, which reduces the chance of air bubble on the working surface of your mold. Once you add the silicone thinner to the silicone base, you weigh out the catalyst to match the weight of the silicone base and silicone thinner together. If you start with 500 grams of silicone and add 50 grams of thinner, the total weight is 550 grams and you would then add 55 grams of catalyst. You don’t want to add more than 10% silicone thinner because, while it reduces viscosity, it also reduces tensile and tear strength.

For example:

5% silicone thinner reduces viscosity by 24%, the shore hardness by 13%, the tensile strength by 13%, and the tear strength by 12%

10% silicone thinner reduces viscosity by 45%, the shore hardness by 23%, the tensile strength by 18%, and the tear strength by 16%

  • Silicone Thickener:

The silicone thickener is similar to the silicone thinner in that it is added to the silicone base (part A) to control the viscosity. The thickener is used to make a pourable silicone a brushable silicone.

To use the silicone thickener, add approximately 1% by weight to the silicone base and mix well. Again, you must use a digital gram scale in order to ensure correct weight. Add the catalyst to the TOTAL weight of the silicone base and silicone thickener. Mix well and brush on to your original. 1% is the general starting point for a brushable material. You may want to adjust up or down, depending on your application. The thickener does not affect the cure time or the durability of the finished mold.

These are the basics of working with silicone additives for tin catalyzed silicone. If you have any questions about this topic or any other, please send us an email at To see our product line and how-to videos, visit our website at









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