Gelcoat is the resin finish applied over any fiberglass structure to give it a protective coating. For a boat, this finish protects the hull from leakage by covering the porous fiberglass and creating a strong glossy shield. Whenever any area of the gelcoat of a boat is damaged either by scuffing or nicks and gouges, it should be repaired to prevent damage to the hull and to keep water from leaking into the boat's interior. The best method for applying gelcoat is with a spray applicator, but if none is available, it is possible to brush on the gelcoat.
Clean the area to be painted or repaired completely. Remove any old coating with acetone.
Use a full brush loaded with gelcoat. Using too thin an application will cause the brush to leave bristle marks along the surface.
Use fine grain sandpaper to smooth the surface after each application. No matter how much gelcoat you use on the brush, you are going to have some brush marks. These should be removed with fine sandpaper between each coat.
Apply several layers of gelcoat for a thick, firm surface.
Allow each coat to dry completely before the next one is applied. Brushed-on gelcoat does not dry as well as sprayed on applications and can be more brittle when finished.