How to choose and apply a long-lasting clear finish.
What do fancy wooden boats and beautiful wooden front doors have in common? They both need a clear finish that can really stand up to the elements. Sunlight, water, extreme temperature changes and abrasions are bound to occur in both situations. Most clear coatings just can’t take that type of abuse and will fail in a year or two, allowing discoloration and damage to the wood.
Clear exterior varnishes
Spar varnishes are formulated to be resilient under outdoor conditions. They’re more elastic than regular varnish, so they’re less likely to crack as the wood continuously expands and contracts with changing outdoor conditions. Some spar varnishes also contain very effective UV inhibitors. The finishes with the best track record for exterior use come from the marine industry, where spar varnish originated.
Spar varnishes are usually glossy. A glossy sheen looks good on a wooden boat and also tells the owner when it’s time to apply a fresh coat of finish. A loss of sheen indicates that the finish is beginning to degrade and it’s time for recoating. Some brands offer a lower sheen option, usually recommended for application as the topcoat over several coats of high gloss for the best results.
Most of the spar varnishes at hardware and paint stores will work well on an exterior door that’s tucked under a front porch, where the sun and rain are kept at a distance. But for a door that will be fully exposed year round in a harsh climate, a high quality oil-based marine-grade spar varnish will provide both good looks and a long service life before it requires maintenance.
Tung oil and phenolic resin are the key ingredients in many marine-grade spar varnishes. The most technically advanced marine spar varnish formulations also contain ultra violet light (UV) inhibitors. UV inhibitors reduce the effect of the sun’s rays by changing UV light energy to heat, which can then dissipate without harming the finish or the wood beneath it. These are expensive ingredients and are reflected in the cost. A quart of the good stuff can cost $40 or more, but this is a small price to pay compared to a finish that fails after a year or two.
Unfortunately, it’s almost always impossible to identify the ingredients by reading the label (or even the Material Safety Data Sheet), because they’re rarely listed. Some brands do list ingredients on their websites or other literature. Be aware that labels can carry misleading claims, such as “maximum UV protection” or “UV stabilized,” even though the product doesn’t contain any significant UV inhibitors. That’s why the spar varnishes that I trust the most are specifically designed for and tested over time in the most extreme conditions. These premium spar varnishes are also formulated to level well (reducing brush marks) and to allow ample working time. Finding such spar varnishes usually requires shopping at a marine supply store or in the advertising pages or websites of boating magazines.
Applying spar varnish
I’ll start by preparing this new entry door for finishing by wetting it to eliminate hidden marks (Photo 1), sanding it to remove unsightly factory sanding marks (Photo 2)and standing it, so both sides and all the edges can be finished at the same time (Photos 3 and 4).
Next, I’ll apply a dark brown oil-based pigmented stain (Photo 5 and Sources). Pigmented stain adds more than color—the pigments provide some UV protection for the wood as well.
When the stain is thoroughly dry, I’ll apply three coats of Epifanes clear high gloss varnish—my favorite premium marine spar varnish (Photo 6). To give the door a hand-rubbed appearance, I’ll switch to Epifanes wood finish matte for the final coat.
This finish—five layers all told—will provide many years of protection before requiring maintenance.
I’ll use a 2″ natural bristle varnish brush and small round sash brushes to apply the varnish (see Sources, below).
The 2″ brush will do the bulk of the surface (Photos 7–10); the sash brushes provide better control on the moldings around the raised panels (Photo 11).
They’re also the best tools for removing drips (Photo 12). Avoid multiple cleanings by submerging the brushes’ bristles in paint thinner between coats.
Here are some supplies and tools we find essential in our everyday work around the shop. We may receive a commission from sales referred by our links; however, we have carefully selected these products for their usefulness and quality.