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Coating Function
A finish coating—paint or varnish—over an epoxy barrier coat protects the epoxy from sunlight as well as decorating the surface. In doing so, the finish coating extends the life of the epoxy moisture barrier, which in turn, provides a stable base that extends the life of the finish coating. Together, the two form a protective system far more durable than either coating by itself.
Protection from sunlight is a primary consideration in the selection of a finish coating. Long term UV protection of the barrier coat depends on how well the finish coating itself resists UV and keeps its pigments, or its shield of UV filters on the surface of the epoxy barrier coat. A high gloss finish reflects a higher proportion of the light hitting the surface than a dull surface. All other things being equal, a white (especially a glossy white) coating will last the longest.
Coating Compatibility
Most types of coatings are compatible with epoxy. Thoroughly cured epoxy is an almost completely inert hard plastic. Most paint solvents will not soften, swell or react with it.
One-part polyurethanes and polyester gelcoat can be affected by epoxy amines and if used must be applied when the epoxy is thoroughly cured, generally after two weeks at room temperature. A thorough cure can be achieved much quicker with elevated temperature post curing. Post curing will also improve epoxy’s thermal properties and is recommended if dark paint is to be applied over epoxy.
Finish Coating Types
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.
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.
