Q: We just stripped six layers of paint off the front door of our 1820s house. I think the door is made of chestnut. We don’t want to repaint it, and we don’t like polyurethane. What’s the best natural finish for wood? Will shellac hold up? — P.K., Katonah
A: Shellac was the finish of choice in days of old, and dates back more than 3,000 years, if Wikipedia has its facts straight. It’s made from a resin that the Asian red lac bug secretes onto the bark of trees. Shellac is plastic’s natural precursor, with an astonishing array of uses, from strengthening ballerinas’ pointe shoes to glazing fruit as well as being a protective coating for wood. It comes in several warm colors in the yellow-red-brown spectrum, and it’s easy to apply. Although shellac is a good moisture barrier, it’s brittle, and is likely to crack and peel on an exterior door unless the door is well protected from the sun and weather.
Linseed oil (made from flax seed) and tung oil (from the nut of the tung tree), or some combination of the two, are possibilities. You can brush or rub them on. The boiled version of linseed oil dries faster, but pure linseed oil provides better protection. Danish oil, a blend of natural oils and resins, wears better than pure linseed or tung oil, and is more water resistant. It also comes in a range of tints. Any oil will darken the wood to some degree and you’ll need several coats.
Heritage Natural Finishes, a Colorado company, offers biodegradable, non-toxic oils along with a concentrated finishing oil that “works wonders” (the company claims) on reclaimed or absorbent woods. For extra protection, you can top it with an application of their liquid wax, and buff to a soft sheen.
If you don’t mind a glossy finish, a marine spar varnish would be the most durable choice. True spar varnishes, formulated with tung oil as a base, are expensive but worth the extra bucks; cheaper versions from the big box stores are polyurethane based.
Whether you go with shellac, oil, or spar varnish, expect to reapply the finish every year or two on exterior wood. One benefit of natural finishes is that you can do spot repairs on scratches, scuffs, or worn areas, something that poly-whatevers don’t allow.
A word of caution: Don’t leave an oil-soaked cloth folded or rolled up — it can spontaneously combust as the oil dries. Best to leave used cloths lying flat outside to dry before you discard them.