In general, for brushing and rolling applications, TotalBoat Gelcoat products do not need thinning.
For spraying applications, gelcoat should only be thinned with styrene, and only as needed. The percentage of styrene needed depends on ambient conditions and the spray equipment you're using. Thin gelcoat with styrene, up to a maximum of 15%. Adding more thinner than this amount can affect the color of the cured gelcoat.
Even though it's one of the most commonly used products used to thin gelcoat, acetone is not recommended. This is because it will lower the viscosity, but has a strong tendency to change the gelcoat color during curing.
Specialty thinning products mentioned in the TotalBoat Gelcoat Technical Data Sheet include Sea Hawk Patch Aid 8185. I you use this product instead of styrene, follow the specific directions on the product's label regarding thinning, catalyzing, and application.