In a nutshell, there are a lot of variables. Cure temperatures and catylist ratio, quality of the gelcoat, thinners, additives, etc. will affect porosity. Gelcoats are porous inside with a non-porous skin on the outside and they are really intended to be applied in a female mold. When it's done this way, the outer surface (in contact with the smooth and fair mold that won't require refinishing) is non porous and last a long time. Once the sun and environment finally gets through that outer layer of gelcoat, the boat becomes harder to keep shiny as the porosity makes the gelcoat more susceptible to breaking down and it will not hold a finish as long.

When we gelcoat on an exterior surface, you have to sand through the outside layer and polish to remove the orange peel to achieve a smooth surface. Once you do that, you remove the durable solid layer and expose the more porous layer that won't hold a finish as long.