We’re entering mid-summer and you may be thinking of applying a clear finish to some project that will be exposed to sunlight outside. What finish should you use? Be careful. All finishes marketed for exterior surfaces aren’t the same, no matter the claims of manufacturers.
The distinction in exterior clear finishes is whether they contain UV-resistant additives, called “UV inhibitors” or “UV absorbers.” And whether they contain enough of these additives to be effective.
Exterior finishes that contain these additives are much more expensive than those that don’t, but they are the only ones that hold up in sunlight. Exterior finishes that don’t contain these UV absorbers still work well in shaded areas.
Exterior finishes generally go by the name “spar,” as in “spar varnish.” Sometimes, a distinction for UV resistance can be made between common spar finishes (without UV inhibitors) and “marine” or “boat” finishes (with UV inhibitors). But there is no industry agreement on the naming, so you’re left with price and where you buy the product being the best indication of which you’re getting.
The spar finishes you buy at home centers and ordinary paint stores generally either don’t contain the absorbers or contain too little to be effective against sunlight. To find exterior clear finishes that contain an adequate amount of the UV-resistant additives, you usually have to go to a marina in a coastal area, or find one of these marinas online and purchase by mail order.
Even with a finish with good UV resistance, if the finish is exposed to a lot of sunlight, you still need to maintain it a lot more than you are used to with a finish indoors. The sun will dull the finish, then begin breaking it down. This happens on the surface first, of course, so the easiest procedure to follow is to sand through the damage, then apply another coat or two of the finish. You don’t necessarily need to strip the finish all the way to the wood. It’s less work to keep up the finish than to replace it.
This is the procedure most wooden boat owners use.
– Bob Flexner