Gelcoat is a resin system that is filled with colorants and thickening agents and applied to a mold surface to form the cosmetic and weathering coating of a composite part. Gel coat is typically the first layer of a part to be applied. Correct application of gelcoat is critical to get the optimum performance from it.

Gelcoats are based on a thermoset resin that requires an initiator and proper temperatures to cure thoroughly.  Incomplete cure will result in poor durability of the gelcoat.  This is often witnessed as premature fading, dulling of the gelcoat surface, or chalking.  There are 3 variables that affect proper cure: temperature, film thickness, and initiator level.

Temperatures should be between 60-80 degrees F.  with an optimum range being between 70-75F.  We like to use an infrared thermometer to check the temperature of the mold surface and gelcoat.  We use the infrared and a wall thermometer to check ambient temperature of the workspace.  Keep in mind that molds stored in areas that are not climate controlled may not reach room temperature for quite some time.

Gelcoat should be initiated (catalyzed) to set up to the point that you can touch it with a glove and not have it come up on your finger in 45-60 minutes from application.  Generally speaking and depending on the temperature a catalyst ratio of between 1.2% to 3% will give you the proper cure rate..  At 77F 1.8% MEKP will provide a gel time of 10-17 minutes. Keeping good notes will help you build a database of initiator levels for temperatures that you encounter frequently. Gelcoat that is over-initiated can result in excessive shrinking and premature release from the mold.