Clean your surface well and then mix and apply marine or fiberglass polyester resin as per the manufacturer's instructions. Shape the resin close to the final shape you desire.
Cure the resin for a minimum of six hours at room temperature. Feel the surface of the resin. If it feels smooth and slick to the touch, then it is properly cured -- if it feels tacky or sticky, then allow it more time to cure.
Sand the surface of the cured resin with 120-grit sandpaper. Do not sand heavily. Sand to remove any fibers or other imperfections. Shape as you sand.
Sand with 180-grit paper for the final sanding to smooth the surface of your resin until it is virtually as smooth as glass. Wipe the sanding resin down with a tack cloth and then paint if desired.