Why Paint
If fixing for resale, if the decks are in good shape, paint would work. If keeping it, re gelcoat. Paint scratches too easily. If the deck needs chips filled ect. then paint or regel is it. If the decks are ok then just clean them. Cheapest rust stain remover that works on fiberglass is CLR, you can find it at hardware stores/Walmart/etc. A good fiberglass polish will take out small scratches and clean up a lot of stains. Coat with a good wax. A lot of elbow grease will be needed but not expensive. To spray a new coat of gel coat on is about as easy as painting, but you have to clean up the equipment fast before the gel starts setting with acetone. Good prep work is the key for either paint or gel coat.