March 23, 2017

Sometimes, when working with new materials or working familiar materials in a new way, you run into issues. We previously wrote a post about common problems with curing,   This post will focus on issues that might have a curing component (soft spots, sticky areas, etc.), but mostly on problems with the materials acting in unexpected and unwanted ways. These aren’t all the problems that could come up, only the more common problems and issues. You can always email us at if you have questions.


  • The silicone has partially cured, but is still tacky  First, make sure the silicone has had adequate time to cure by checking the cure time on the Technical Data Sheet. The Technical Data Sheet can also tell what causes cure inhibition for that silicone. Second, double check that you shook the catalyst for 60-90 seconds before measuring and mixing.  Insufficiently shaken catalyst is a very common way to get tacky silicone. Also, if it is too dry or too cold, that can cause the curing process to take longer.
  • Soft/partially cured spots  This is almost always from material that hasn’t been mixed well enough. However, if you give the silicone a few extra days, the soft spots will usually cure in the end.
  • See All Silicone Mold Products

  • Silicone is very brittle  Too much catalyst will make your silicone brittle. Check the Technical Data Sheet and make sure you are adding the correct amount of catalyst.
  • Air bubbles on the working surface of the mold  A few things can cause this. 1. You poured the silicone directly onto the piece. To avoid air bubbles, pick a corner of the mold box to pour and don’t pour the silicone directly on the original. 2. You poured the silicone too quickly. Pour “high and thin” as this gives the silicone time to flow around the original so pockets of air don’t get trapped. 3. There is a lot of fine detail on the original. Lots of detail gives air bubbles lots of places to cling to. Try brushing some of the silicone on the original before you pour the silicone. This method gets the silicone into crevices and helps break the surface tension that lets bubbles stick. If you are using brushable silicone and get air bubbles on the working surface of the mold, you applied the first coat too thickly. The first coat should always be the thinnest coat to ensure no air bubbles.

 Air bubbles on the working surface

  • Brushable silicone is sagging off the original The silicone has been applied too thickly. Apply the silicone more thinly, in several coats, in order to build up an appropriate mold thickness.

 Saggy silicone.

  • Silicone is sticking to the original  Silicone will chemically bond to materials that contain silica. The most common silica containing things are silicone, glass, quartz, and some pottery glazes. If you have any doubts about what your original is made of, do a small test patch on an unobtrusive area of the original piece before doing the full mold. Silicone can also mechanically bond to very porous materials. If you have a very porous original (old plaster, wood, or concrete are common porous materials), paint it first. If you don’t paint it first and the silicone sticks, patience and elbow grease can usually get the silicone off. Just peel it off very slowly to avoid damaging the original.


  • Air bubbles  It could be a few things; 1. The latex was brushed on too quickly. Slow down and brush it on your piece a little more slowly. 2. The latex was applied too thickly. Nice thin, even coats will yield the best results. 3. Using a heat gun on either too high of a setting, too closely to the mold, or holding it too long in one spot. If you are using a heat gun to speed up the dry time, use the low setting, hold the gun at least 8 inches away from the latex and keep it moving at all times.

See All Latex Mold Products

Urethane Rubber:

  • Lots of bubbles, a foamy gray appearance  Moisture contamination. Urethane does not like contact with moisture before it’s cured. Don’t use paper, waxed or wood mixing containers or utensils and don’t mix the material on a rainy day or when the humidity is over 50%.
  • Urethane is sticking to the original or the mold box  Urethane likes to bond to most things. Check the Technical Data Sheet for a full list of what it won’t bond to. Use silicone paste (which comes with the 1/2 gallon and 2 gallon kits that we sell) or petroleum jelly on the original piece and the mold box.
  • See All Urethane Molding Products

  • Ripples or waves in the material or air pockets forming around the original The material is not being poured before it begins to gel. Mix and pour the urethane within 3 minutes so it has time to flow before starting to gel.
  • Soft spots  Material wasn’t mixed well enough. Don’t forget to scrape the sides and bottom of the mixing container and mix very well.

Urethane Casting Resin:

  • Bubbles, foamy appearance  Moisture contamination. Urethane does not like moisture before it’s cured. Don’t use paper, waxed or wood mixing containers or utensils and don’t mix the material on a rainy day or when the humidity is over 50%.
  • See All Urethane Casting Resins

  • Extra Oily  Too much Part B in the mix. Make sure that you are measuring 1:1 by volume. To take care of the extra oil, dust with or submerge the part in talc, wait 12-24 hours and then wash the part with dish soap and water.
  • Soft spots  The urethane casting resin wasn’t mixed thoroughly. Don’t forget to scrape the sides and bottom of the mixing container and mix well.


  • “Oil and water” or blotchy appearance  This is usually caused by an oil based stain on the wood or actual oil stains on concrete. Try to remove all large oil stains from the concrete before applying the epoxy. If your wood is colored with an oil based stain, do thin brush coats of the epoxy, followed by a light sanding and then wipe with acetone before the next coat. Repeat these steps until the epoxy looks even.  Then, do your pour coat to the desired finish (but no more than 1/4 to a 1/2 inch at a time, depending on the surface area).
  • Lots of small bubbles  The substrate is either very porous or you are mixing with an electric or drill mixer. For porous wood or concrete, do a brush coat or two before doing the pour coat. Always mix the epoxy by hand to avoid whipping air into the mix and to avoid the friction from setting off the cure reaction early.
  • See All Epoxy Resin Products

  • Many small or large bubbles that form about 10-20 minutes after pouring  This occurs when you pour the epoxy too thickly.  Too thick a pour will have the epoxy boiling. You can sand out all the bubbles and then do thinner pours to avoid repeating the problem.
  • Waves or ripples in the epoxy  Either the epoxy is being poured after it has started to gel or you are holding a propane torch or heat gun too close to the surface of the epoxy. Also, please remember that a hair dryer is not the same thing as a heat gun and should not be used on the epoxy because it will make waves in the epoxy. Mix the epoxy well and pour it within 5-10 minutes (quicker mixing and pouring for larger batches) to avoid the epoxy curing while you are still removing waves or ripples.
  • Epoxy cured cloudy white  The epoxy got wet while it was curing. You can sand off the white and pour or brush coat again. Always check the weather forecast before doing any outside work with the epoxy because it needs to be kept dry while curing.
  • Epoxy got super hot and cured in the mixing container  Possible causes: 1. You mixed too large a batch. 2. The ambient temperature was too high. If the temperature is over 75F, mix smaller batches than you normally would because the epoxy will cure faster as the ambient temperature increases over 75F. 3. You used a drill or electric mixer and the friction kicked off the cure process. Always mix the epoxy by hand.

These are some of the more common problems that can arise when using our products. For more information on our products, check out our website, or send us an email at


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