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Gelcoat Application - Thickness, Initiator Level, Temperature

Gelcoat Header

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.

Our standard gelcoats are designed to cure in films as thin as 12mils (0.012 inches); however, we recommend that you apply them in coats of 18-20 mils to get the best cure, hide, and print block.  As the gelcoat cures, it develops heat that helps it complete the cure cycle. If your coating is too thin and the temperature is too low, it will not cure properly.  Gelcoat can be applied in coats up to 24mils.  For applications under the waterline a thickness of 20-24 mils aids in blister prevention and is better than thinner layers. Thicker layers are more prone to cracking under stress.

Styrene vapors will inhibit and slow the cure of gelcoat.  Because styrene is heavier than air it will pool at the bottom of a mold cavity and slow down the cure of the gelcoat located in the bottom or lower parts of a mold.  If you have the ability to do so, rotate the mold so that while the gelcoat is curing the styrene vapors can pour out of the mold.  If you do not have that ability, make sure to check low spots in the mold as well as high spots for the cure rate of the gelcoat.  Slow moving fans can also be used to move the styrene out of the mold cavity.

Gelcoat Mil Gauge Gelcoat Mil Gauge, measures wet film thickness.

Gelcoat Mill Gauge in Use
The thickness of the gelcoat is between the last tooth to come up with film on it and the next clean tooth.  In this picture the largest wet tooth is the 15mil tooth and the next clean tooth is the 20 mil tooth, the gelcoat is between 15-20 mils thick (0.015-0.020”) Check in more than one spot for consistency.

You may also be interested in:

MEKP Chart
Gelcoat Calculator- Estimate how much gelcoat you’ll need
Spraying Gelcoat Video


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Fiberglass Supply 
 11824 Water Tank Rd. Burlington
 Washington, 98233 USA
Phone; (509) 493-3464
Fax(360)757-8284
 e-mail support@fiberglasssupply.com