Painting Over Gelcoat

Gelcoat is a resin used in the building of fiberglass bodies for boats and other vehicles. It is used to keep the fiberglass from sticking to the mold during production and binds to the fiberglass, forming a coating after the boat's body is removed from the mold. While it is colored and has the appearance of paint, it reacts to sunlight, eventually turning it cloudy or fading it over time. One solution to this problem is to paint over it, but painting presents its own issues.

  1. General Issues

    • Extreme temperature variations can cause fiberglass to expand and contract. Dark colors absorb more heat from the sun than lighter colors and should be avoided. They can cause the underlying fiberglass to shrink and eventually crack the paint and distort the entire surface. Because urethane paints are highly toxic, sprayers should only be used indoors to avoid violating pollution laws. Spraying outdoors is inefficient anyway; half the paint will blow away in even light breezes. Paints are meant to be applied at roughly room temperature, about 70 degrees Fahrenheit. Extreme temperatures or high humidity will harm the paint job.

    Preparation Issues

    • The gelcoat must be cleaned, sanded and primed before paint is applied. Be very careful not to sand through the gelcoat layer when removing gouges and chips; if you do penetrate that layer, apply more gelcoat before painting. All caulking must be removed from areas to be painted as caulk will cause the paint to crack and flake off. All hardware, wood trim, plastic parts, window frames and other non fiberglass parts should be removed and, if necessary, painted separately. Any deep gouges should be patched with body putty before sanding and sealing.

    Painting Issues

    • Use a high-volume, low-pressure sprayer to apply the paint. Lesser sprayers will lose far more paint in over spraying, particularly outdoors, than HVLP guns. Check the gun carefully before beginning; if it is not clean, have it cleaned and optimized by a professional to save on over-sprayed paint. Examine the paint job carefully and buff any flaws with #2000 grit sandpaper and water. Spray the area with a hose while you are sanding to polish dull or streaked areas. Repainting won't cover the flaws.

    Safety Issues

    • Urethane paints are highly toxic. Wear breathing equipment while spraying, even outdoors. Masks and cartridge-type respirators are not adequate. Indoor areas should be well-ventilated to be extra safe, but quality breathing equipment is still crucial.