Finish Coating Types
Latex paints are compatible with epoxy, even partially cured epoxy, and they do an adequate job of protecting the epoxy barrier from UV radiation. In many architectural applications, latex paint may be the most suitable coating to use. Their durability is limited.
Alkyd finishes - enamel, alkyd enamel, marine enamel, acrylic enamel, alkyd modified epoxy, traditional varnish and spar varnish - offer ease of application, low cost, low toxicity, and easy availability. Their disadvantages are low UV resistance and low abrasion resistance.
One-part polyurethanes offer easy application, cleanup and better properties than alkyds. They are also more expensive and some may be incompatible with amine cure epoxy systems such as WEST SYSTEM epoxy, although 207 Hardener may offer better compatibility. Be sure epoxy has cured thoroughly. Make a test panel to assure compatibility.
Epoxy paints are available in one-part and two-part versions. Two part epoxies offer many characteristics similar to the higher performance polyurethanes. They are durable and chemical resistant, but offer limited UV protection compared to the linear polyurethanes.
Two-part linear polyurethane (LP) paints offer the most durable protection available. LP's are available as pigmented or clear coatings and offer excellent UV protection, gloss retention, abrasion resistance, plus compatibility with epoxy. However, compared to other types of coatings, they are expensive, require more skill to apply and present a greater health hazard, especially when sprayed.
Bottom paints are available in a variety of formulations. Most bottom paint systems are compatible with epoxy and can be applied directly over a prepared epoxy barrier coat. If you are unsure of compatibility or have curing or adhesion problems with a specific bottom paint, use only a primer recommended for that bottom paint over the barrier coat. Follow the recommendations given for preparation of fibreglass surfaces. Other paints, including marine LP's and primers, are not recommended for use below the waterline.
Primers are usually not needed to help a paint film bond to epoxy, although interfacing primers may be required with some specialised bottom paints and high-build primers are useful for hiding scratches or flaws in the substrate. If the instructions on your paint or varnish recommend a specially primed surface, follow the recommendations given for fibreglass preparation. Self-etching primers do not work well on an epoxy coating because of epoxy's chemical resistance.
Polyester gelcoat is a pigmented version of the resin used to build fibreglass boats and other products. Gelcoat is sprayed into a mold before the glass fabric and resin are applied to provide a smooth pre-finished surface when the part is removed from the mold. It is not often used as a post-production finish coating, but it can be applied over epoxy and is useful in some repair situations. Epoxy must be cured thoroughly.
Follow all instructions from the coating systems manufacturer. It is a good idea to make a test panel to evaluate the degree of surface preparation required, and the compatibility and handling characteristics of the finish system.