Do you need bottom paint?
Sounds like you need fiberglass repair and maybe a chemical hull cleaning (try Zing) to get rid of the stains. Antifouling paints protect your hull from growth, they don't fix scratches, etc. Look up the list of topics and I have written a lengthy discourse on bottom paints. But a Sunfish can be pulled up on the beach, put atop the car or a trailer and bottom paint wouldn't be needed I wouldn't think.