Varnish and various wood oils have been the traditional transparent wood finishes on boats for hundreds of years. Their lack of pigment allows the beauty of the wood grain to show through, while protecting the wood from the ravages of sunlight, saltwater, dirt and abrasion. The finish achieved after careful application can vary from practically invisible protection for interior and exterior wood, to a glossy fine-furniture coat.
Assuming you want to protect and beautify your boat's exterior wood, how do you decide between the incredible (dare we say baffling?) variety of finishes? Knowing full well that this is as big a minefield as debating the benefits of power vs. sail, we nonetheless courageously offer the following observations.
Wood Oils
Oils generally maintain the original look and texture of the wood more than the other finishes because they penetrate deeply into the wood fibers and do not create as much of a surface coating. Oils are available in colors ranging from water clear to gold to dark brown. As with any wood finish, multiple coats generally result in a more uniform finish and greater longevity. We find oil to be the easiest to apply, since surface blemishes are not as apparent in the final outcome. This does not mean that slapping four coats of oil on dirty teak will produce a Bristol finish. You will also find that oil’s thin consistency makes masking to protect surrounding gelcoat and painted surfaces almost as important with oil as with varnish. Clean, sanded teak with several coats of oil can provide 3–6 months of beauty and protection.
Varnish
Varnish is a complicated finish consisting of oil, solvents, thinners, resins, dryers and additives. By varying the proportions of the components and by adding UV inhibitors and other secret ingredients, manufacturers create varnishes with widely varying characteristics. Varnish can have a gloss or matte appearance, can be formulated to be extremely hard for walked-on surfaces and can vary in color.
The two most common types of varnishes are natural resin varnishes such as tung oil and oil-modified polyurethane varnishes. Natural tung oil varnishes are excellent for interior or exterior use and offer the classic golden look. Oil-modified polyurethanes tend to be more clear, allowing the color of the wood to shine through. Wood subtly expands and contracts with even imperceptible changes in temperature and humidity. Premium varnishes cope with this dimensionally unstable material by using high quality oils and resins and higher percentages of solid ingredients; these provide a longer lasting, more flexible finish with a higher gloss. A good base, regularly and diligently maintained, will give the ultimate in appearance, longevity and protection. Despite the initial effort (up to ten coats are common) nothing looks better than exterior wood finished with gloss varnish.
Synthetic Wood Finishes
These satin coatings gained popularity with cruisers in the tropics looking for UV-resistant, low-maintenance brightwork. They are remarkably durable, easy to apply and look relatively good. We say relatively good because some of these products, like Cetol Marine, tend to have a pigmented appearance, with an orangish brown cast. This is caused by synthetic iron oxide pigments that protect the wood's lignin (a key component in the strength of wood) which is degraded by UV light exposure. Full gloss and depth take a back seat to ease of application and fast drying times with these finishes. Cetol comes in four varieties, with the Natural providing a more golden color resembling the real look of raw teak. Typically, you can apply one coat of Cetol every 24 hours. Overcoat any of the other Cetol finishes with the Gloss for a shiny exterior finish. When applying Cetol Gloss, masking surrounding areas is highly recommended.
Further Reading: How to Apply Teak Oil