Scuff the fiberglass to be painted using fine-grit sandpaper and a palm sander. Sand any rough spots on the fiberglass until the entire surface is smooth and scuffed.
Plug in the air compressor, and blow all the dust off of the fiberglass. Clean the surface using a rag and acetone. This removes any grease or oil residue that can cause the gel coat to bond poorly or not at all.
Cover any decals or windows with painter's tape. This prevents overspray from getting in the wrong places.
Mix the gel coat and hardener as directed using a small bucket and a stir stick. Add the paint to the spray gun as instructed in the spray gun manual.
Spray the first coat of gel coat onto the fiberglass in a heavy mist. For the best results, apply the gel coat as evenly as possible. Let the first coat begin to harden, and then apply the second coat.
Spray the second coat of gel coat onto the fiberglass, solidly covering the surface. Continue to spray as consistently as possible to avoid any runs. Let the gel coat cure until it's fully hardened.