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Curing Issues- Trouble Shooting

January 13, 2017

At some point, everybody mixes up some silicone or epoxy or urethane and it just doesn’t cure right. There are many things that can affect the curing process. This blog post isn’t meant to cover every thing that could possibly go wrong. These are some of the most common reasons for our products to have issues curing. As always, if you have any questions, send us an email at [email protected] and we would be happy to help you.

 Curing problems common to all materials:

  • Ratios- Was the correct amount of Part A and Part B (or catalyst) used? Some materials need to be mixed by weight, some need to be mixed by volume. The Technical Data Sheet and the label on the front of the bottle will tell you the correct mixing ratio.
  • Mixing- Was the material well mixed? Our materials should be mixed by hand, using a wooden paint stir stick (silicones and epoxies) or a plastic mixing utensil (urethanes), taking care to really scrap the sides and bottom of the mixing container.  If your material cures hard in some spots and is tacky or soft in other spots, it is almost always because the material wasn’t mixed well enough.
  • Use by date- What is the use by date on the container? As we’ve covered in other posts, some materials go bad more quickly than others, while some remain viable, under certain conditions, past the use by date. Always check the use by date if the product has been sitting on your shelf for a while and mix a small test batch if it is past the use by date.

The use by date is shown here on the fourth line of the label, under the cure time.

Curing problems common to silicone:

Tin catalyst silicone (128,125, and 115 translucent silicone):

  • Dry weather- Tin catalyzed silicones are condensation cure, meaning they pull moisture from the air as part of the curing process. If the weather or environment is very dry, it can affect the cure time. If you can increase the humidity in the area where it is curing, that will help.

 The humidity is at 68%, which makes it a good day to work with tin catalyst silicone.

  • Cure inhibitors- Sulfur containing clay and latex gloves are the most common cure inhibitors for these types of tin catalyzed silicones. Check that any clay you are using does not contain sulfur and make sure the gloves you are wearing are not latex gloves.
  • Catalyst issues- For the 128 pourable and brushable silicones as well as the 115 translucent silicone, the catalyst will separate if it sits for more than a few hours. Therefore, it needs to be shaken vigorously before use.
  • Additives- Different additives (silicone diluent, silicone thickener, silicone accelerator) need to be added to either the silicone Part A or the catalyst to work. The Technical Data Sheet will tell you which part and how much to add.

Platinum catalyst silicone (Food Grade silicone, silicone putty, and 150 potting and high temp):

  • Cure inhibitors- Platinum catalyzed silicone has many cure inhibitors. The most common cure inhibitors for platinum catalyzed silicones are: tin (tin catalyzed silicone), sulfur, latex, alcohol, ethanol, methanol, ethyl acetate, vinyl acetate, melamine, amines and nitriles. Most of these are listed in the Technical Data Sheet. If you have questions, please give us a call.
  • Catalyst- Always shake the catalyst thoroughly before use.
  • Cold weather- platinum catalyzed silicones are addition cure and can be affected by lower temperatures. Warm up your work area if possible to help it cure in a timely manner.


  • Cure inhibitors- Once again, sulfur containing clay can cause curing issues.
  • Application- Latex needs exposure to air to cure. Too thick of a coat will cause the latex to take a very long time to cure, and if it’s much too thick, it might not completely cure at all. Apply the latex in very thin coats for best results.

 A nice thin, quick curing coat of latex.

 Too thick a coat of latex, lots of bubbles, it will take forever to cure, and it might not cure all the way through.

  • Cold weather- Cold temperatures can also slow the cure time. A heat gun can be used to speed up the dry time, just make sure to use it on the lowest setting, keep it at least 8 inches away from the latex, and keep it moving. You can also set the piece out in the sun or raise the temperature of the room where you are working  to 72F.

Urethane (rubber and casting resin):

  • Cure inhibitors- Again, sulfur containing clay can affect the curing process.
  • Shake Well- Both the A and B sides of these materials should be shaken well prior to mixing, especially the urethane rubber.
  • Cold weather- Urethanes do cure more slowly in colder weather. If possible, warm up your working area to 72F or set the bottles in the sun for 10-30 minutes before use. If pouring the urethane into a mold or form, make sure the mold/form is warmed up as well.
  • Batch size- Smaller batches (especially smaller than 6 oz) cure more slowly than larger batches, as do thinner pours that have a lot of surface area to let off heat. Warm the material, warm the work area and just be patient. It will cure eventually.


  • Double mix and pour- Because both epoxy resin and hardeners are both clear, it can be difficult to tell if the material is well mixed, which is why we recommend using the double mix and pour method. To do this, measure out your epoxy and hardener and pour them into a clean mixing container. Mix well for for a few minutes, taking care to scrape the sides and bottom of the container. Don’t whip or beat the epoxy because that will create air bubbles. Then pour the epoxy/hardener into a new, clean mixing container and mix for a few more minutes. From there, you can pour or brush or roll the material on as desired.
  • Cold weather- The epoxy curing process slows down at 65F and decreases very dramatically under 50F. Warm up the materials and your work area to 72F and be patient. It will cure eventually.
  • Batch size – Smaller batches (under 8oz) cure more slowly. Same as with cold weather, warm things up or be patient.

These are the more common issues that can affect the curing process for the various materials we carry. For more information, along with Technical Data Sheet for all the materials we have, please go to our website,



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