How long does gelcoat need to cure before it can be sanded?

Gelcoat can be sanded as soon as it is hard enough to sand to a fine powder, and does not feel gummy under the sandpaper. The amount of time it takes to get to this point is a variable that is dependent on the ambient temperature, how thick the gelcoat was applied, and how much MEKP catalyst was used. If colorants, solvents, or anything else was added to the gelcoat, it can slow the cure rate even more.