Painting a fibreglass boat can improve its life and appearance, as some paints can protect the hull even better than the fibreglass gel coat. But painting a fibreglass hull is slightly different than with a wooden hull, especially with preparation. There are a few challenges you need to overcome. The paint won't adhere well the fibreglass gel coat by itself since the coat is very slick and not porous. There are also very few paints that will work well on a fibreglass hull and can be legally used by unlicensed amateurs (due to their chemical make-up).
Remove all wax from the hull with a commercial solvent. The solvent must be blended to remove silicone if you have used silicone polish on the boat. Apply the solvent generously on a cloth pad, rotating the pad so you always have fresh cloth. Applying the coat twice is a good idea.
Fill any gouges and scrapes on the hull with epoxy putty. This putty often comes in a two-tube container and must be mixed before applying it with a small brush or paint stick. Use enough putty to stand or bulge out from the surface. Sand the putty down with a dual-action rotary or orbital palm power sander.
Apply a special fibreglass primer (also known as a tie coat) that will soften and bond to the fibreglass gel coat. Make sure the primer is compatible with your finish coat paint; the container will tell what paint types it works with. Cover the entire hull with a thin coat using a four-inch brush or roller.
Apply the finish paint using a roll-and-tip method, which requires two people. The first person applies a thin coat with a roller, while the second runs a foam brush across the applied coat to smooth it out. Use a four-inch roller or smaller so you can cover as much of the painted area with a three or four inch brush.
Always wear old clothes that you won't mind getting paint on. You can use a high quality enamel used on yachts instead of a two-part urethane. This type of paint can be safer, but won't last as long.
Use a low-quality two-part urethane paint for the finish coat. It can't contain any dangerous, restricted chemicals like isocyanates. Under EPA regulations, non-professionals cannot use paints with these chemicals. Make sure your brushes and rollers can work with your paint. Sometimes the paint solvents are strong enough to make the brushes and roller covers fall apart. Work in an open, well-ventilated area when using the paints, primers and solvents.