Kinds of Oil Finishes

Oil finishes are referred to as penetrating finishes: the oil permeates the wood by capillary action, forming a continuous film over the surface. Most of the literature lists four kinds of oil finishes: ‘true (or straight) oil’, ‘polymerized oil’, ‘blended oil’, and ‘wiping varnish’. The first three are oils, while the last is not an oil, it is a diluted varnish (which we will cover in a future article).

True oils include linseed, boiled linseed (linseed oil to which heavy metal dryers have been added), tung, and walnut oils. They have a distinctive ‘nutty’ odour.

These oils cure slowly to a comparatively soft and satin sheen. If you apply five or six coats, pure tung oil is supposed to offer fairly good water resistance, but it still scratches easily and doesn’t offer much water vapour exchange protection.

Linseed oil takes forever to dry, and it imparts a yellowish hue to light woods. Boiled linseed oil, available from your local hardware store, cures a lot faster than in its raw form. Tung and walnut oils are commonly used by those desiring a toxic free finish.

If you heat linseed or tung oil to around 500ºF in an oxygen free environment it undergoes a chemical change called polymerization, producing, of course, polymerized oil. The heat treatment increases the oil’s hardness and gloss and speeds up its drying time. Some polymerized tung oils are very thin because of the addition of a high proportion of mineral spirits (up to 50%). When buffed they produce a high lustre and sheen. Once these finishes have cured and the mineral spirits dried off, the finishes are non-toxic. (Note: the polymerized linseed oil is sold as Tried and True’s Danish Oil).

Blended oil finishes contain a true oil mixed with varnish or polyurethane (they may also contain other components: most commonly, mineral spirits and heavy metal dryers). The oil in the finish makes it softer, makes it cure more slowly and makes it cure to a satin sheen, while the qualities imparted by varnish make the finish cure harder, glossier, and increase the protective characteristics of the finish.

Blended oils may bleed finish from the pores back to the surface if you apply too much finish. Better to apply thin coats and wipe well. There are a lot of different blended oils on the market. Many of the products sold as ‘Danish Oil’, ‘Antique Oil’, ‘Teak Oil’ or ‘Finishing Oil’ are likely blended oils. One of the most popular is Liberon Finishing Oil. Another that is becoming popular is Tried and True’s Varnish Oil (available from Lee Valley), which is polymerized linseed oil combined with natural-resin varnish (modified pine sap). Once these finishes have cured and the mineral spirits dried off, the finishes are non-toxic; unless the manufacturer has added metal dryers or other nasty surprises (which may or may not be listed on the product).

The first photo shows four different oils finishes. Notice how thick the Tried and True is in comparison to the others, due to the higher proportion of varnish in the mix. The Liberon is very thin; you can see it’s more quickly penetrating the wood than the other finishes. The linseed oil shows its characteristic yellow tone (which also shows in the Tried and True oil).

Applying Oil Finishes

Oil finishes are about the easiest to apply, which makes them so popular. Until you’re familiar with the characteristics of a product, it’s best if you follow the directions supplied by the manufacturer.

Typically, here is what you do. First, make sure that you’ve finish sanded your wood. Now raise the grain by sponging the surface. Once it’s dry, light sand with one grit higher than the last grit you used in preparing the wood. Flood the surface and let it stand for half an hour. If you notice any dry spots daub on more finish. Now wipe off the surface. Let it dry overnight, then sand with 220 or 320 paper, dust off, and apply another coat. Two or three coats are all you need. With your last coat wet sand using 400 or 600 grit, then wipe the surface and buff it with a cloth. After letting the finish cure for a few days you can apply a wax coat. Notice that I never mentioned the dreaded “D” word: dust.

Because oil finishes penetrate the wood so quickly, and dry very rapidly, you don’t have to worry about dust contamination (or brush marks for that matter). Some people wet-sand the finish with each application of finish rather than wet-sanding only the final coat.

Why Use an Oil Finish?

We’ve already discovered that oil finishes are pretty easy to apply, and the least toxic finishes. They are also very easy to maintain and repair: just wipe more oil on the surface, perhaps with a light wet-sanding or using steel wool if you’ve fine scratches to deal with. Unlike film finishes, that put a tactical and visual membrane between the wood surface and your hand, oil finishes give the most ‘natural’ look and feel. The wood feels like, well, wood. The downside is that you don’t get a lot of protection. Generally, you won’t want to use an oil finish on items that are going to be exposed to a lot of hard use, moisture, heat, or chemicals. So, while you wouldn’t use it for a kitchen or coffee table top, it might just be the ticket for that special display shelf or cabinet, turned bowl, or hand carved sculpture.

Look and Feel

I applied five different oil finishes to a cherry and an oak board following the manufacturers instructions.

The cherry absorbed the finishes much more quickly than the oak. The linseed and raw tung oils gave the darkest finishes. I found that the polymerized tung and blended oils gave pretty similar finishes, being pretty hard to tell apart from one another. These last three finishes looked and felt very nice.

Testing with Water

I was curious to see how these finishes would stand up against moisture. After letting the finishes cure for 3 days I placed a drop of water on the center of each finish and left it there for 60 minutes. No surprise that there was a water mark left on each sample. The water marks were much more visible on the cherry samples than on the oak samples.

The most noticeable was on the raw tung oil finish, and the least noticeable was on the Tried and True Varnish Oil finish. While oil finishes do look good, they are not going to offer as much protection as a film finish.

In my next article I will cover Shellac and French Polishing. Au revoir!