A.Byron Papa responds: I’ve found that almost no clear finish holds up more than a year outside. Clear coatings allow the ultraviolet (UV) rays in sunlight to penetrate, degrading the outermost layer of cells in the wood siding. Add water from rain and humidity (we get plenty of both here in North Carolina) and tiny mildew spores begin growing on the surface of the wood. This further breaks down the bond between the wood and finish. Consequently, with almost any clear finish, you have to recoat each year.

The only thing I’ve found that holds up better is an oil-based clear coating from Sikkens (AKZO Nobel Coating, P.O. Box 5062, Troy, MI 48007; 800/833-7288). This goes on like a varnish, but the surface buildup is 30 to 35 microns thick, which is two to three times thicker than most other coatings. (For the West Coast, where legislation limits...

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