What do I use to thin gelcoat?

The most common product used to thin gelcoat is styrene, but it should be used very sparingly because the color of the gelcoat can shift if too much styrene is added. Styrene is already in the gelcoat, and gives it the easily recognizable smell. 

Other thinners are available on the market, such as Patch Booster and Duratec Clear Hi-Gloss Gelcoat Additive. These products function very differently compared to thinning with styrene, but they are designed to maintain the color of the gelcoat, so it is extremely important to follow all directions carefully.

Acetone is commonly used to thin gelcoat. It is not the ideal product to use, as it has a much higher chance of changing the color of the gelcoat when it cures.