William C. Feist, a specialist in exterior wood finishing and weathering in Middleton, Wisconsin, replies: As you’ve found out, it’s difficult to maintain a natural finish on wood exposed to rain and intense light and heat from the sun. Clear coatings of conventional spar, urethane or marine varnish are not generally recommended because ultraviolet (UV) light from the sun penetrates the transparent film and degrades the wood under it.
Moisture and heat continue the attack on the finish and on the wood. Without care and maintenance, the finish eventually becomes brittle, develops severe cracks and then peels, while the wood underneath the finish darkens and cracks. The more sun, rain and heat the finish gets, the faster the wood and finish degrade. Given all these potential problems, you have some choices for a reasonably durable exterior finish.
The first option would be using a penetrating stain finish, such as Watco (800-635-3286; www.watco.net) exterior stains. Exterior semitransparent penetrating oil-based house stains that contain a mildewcide and that are water repellent would also work well. These penetrating finishes should be applied according to manufacturer’s instructions. Be sure to coat the exterior side and all edges (including top and bottom) before installing the door. These finishes usually require regular maintenance every two or three years, but they are reasonably easy to work with. Just wipe down the door and then reapply the penetrating finish. Be sure that you give the door an annual inspection.
A second choice would be a multicoat varnish system. You can achieve a reasonable service life with varnish if you are willing to work at it (as boat owners do). First, treat the bare wood with a paintable water-repellent preservative such as DAP’s Woodlife (888-327-8477; www.dap.com) on all sides and edges.
Next, apply a varnish-compatible pigmented alkyd-based stain such as a furniture stain. The pigment in the stain helps to reduce UV-damage. Be careful in your choice of stain; some exterior wood stains may not be compatible with the subsequent varnish coats, especially those containing unpaintable water repellents. Lacquer-based stains are not a good choice either. Then, choose a top-quality marine exterior spar varnish (lacquers and shellacs do not make good outdoor finishes because of their sensitivity to water and their brittleness). The first varnish coat should be a sealer made by diluting the varnish 1:1 with solvent, normally mineral spirits or turpentine. Give the sealer coat a light sanding, and then apply a minimum of three coats of varnish with light sanding between each coat.
In marine exposures, six coats of varnish are often applied for best performance. You might consider applying six coats to deal with your severe exposure. But even these measures will require some maintenance every couple of years. Inspect the door annually, and when needed, lightly sand and apply another coat of varnish.
Finally, the best protection of whatever finish you choose is minimizing the heat, light and water that reach the door. Even with your front porch, I suggest that you try to create more shading for the door with a sun shade, with an awning or with some newly planted trees.