There is no defined amount to thin TotalBoat Gelcoat due to the variables involved with applying it. For spray applications, start by thinning 2-5% with styrene, and add more, as needed, for proper spraying, up to an absolute maximum of 15% (the more thinner that is added, the more chance there is that the color of the gelcoat will change slightly). The spray equipment being used and the environmental conditions will dictate how much thinning is required. Gelcoat should not require thinning when applied by brush or roller. If desired, a small percentage (1-3%) of styrene can be added to help with flow.
Other thinners are available on the market, such as Duratec Clear Hi Gloss Additive. These products function very differently compared to thinning with styrene, so it is extremely important to follow all directions carefully. It is not recommended to use acetone for thinning, as it has a higher chance of changing the color of the gelcoat.