The correct way is to use chicken grit,
step 1) fibreglass using matting etc, (leave to dry)
step 2) resin back over what you have done sprinkle the chicken grit evenly over wet resin (leave to dry)
step 3) brush away any excess grit,
step 4) top coat over all the fibreglased areas,
the gel coat is the most important part as this is the bit that makes it waterproof (fibre glass resin is not waterproof)