Preparation makes the difference in any painting project and the key to applying spray paint to a fibreglass boat is preparation. The hull must be cleaned, sanded, primed, sanded yet again and cleaned again prior to applying the final coats of paint onto the boat. Once the preparation is complete, though, the job moves along in much the same way as any other spray painting job, using the same techniques as one would use for painting a car.
Hang heavy plastic sheeting around your work area. The best chance for a good paint job is to paint indoors -- if you have a garage or workshop large enough for the boat. The plastic sheeting will contain the overspray that might change the colour of everything in the work area. Lay a dust sheet (also a plastic sheet) to contain any bits from sanding and keep the spray off the floor.
Wash the boat with a mix of water and mild liquid soap to remove contaminants and dirt that sanding might drive into the finish. Rinse the boat thoroughly and allow it to dry completely.
Set an 80-grit sanding disk into your orbital sander and sand the areas to be painted. Blow the sanding dust away with an air hose and rinse the area to be painted with running water. This is the work that readies the surface to accept the primer coat.
Mix preferably with a mechanical stirrer mounted on a drill to ensure the paint is well mixed. If you're spraying the primer, as well as the finish coat, on to the boat, mix the primer according to the directions provided by the manufacturer for spraying. Apply the primer coat, reducing the primer with the reducer specified by the manufacturer. Rinse the gun and sprayer with the solvent appropriate to the paint once you've finished the primer coat.
Sand again, with 220-grit sandpaper to remove surface imperfections and use an air hose to blow the sanding dust off the hull. Rinse the hull with fresh water from a garden hose and allow the hull to air dry. Mask any areas that require masking, like the exterior of through-hull fixtures, zincs and the water line with masking tape and wipe the hull down with clean rags and a spray paint reducer if you're using a two-part epoxy paint, or with the reducer recommended by the paint manufacturer if you're using another type of paint.
Mix the paint for the top coats according to the directions for spraying that are provided with the paint and are usually repeated on the top of the bucket of paint. Fill the sprayer and apply the paint, continuing past each end of the vessel on each pass. Apply three to five thin, even coats of paint.
A boat can be painted with any paint, even water-based house paint, that you desire.
Wear respirators and follow the other precautions for spray painting. Dispose of dust sheets, rags and waste in an approved manner.