Polyester resin used for laying up fiberglass is not intended to be used as a finish coat.
One reason is the glass fibers will end up exposed and cause serious itching,
the other is that it's not really water tight. It will absorb water over time.
So a finish coating (sealer) is recommended to help prevent these problems.
Gelcoat is the accepted topcoat used by most builders,
but there's nothing wrong with a quality primer, paint, and non-skid additive.