Using Penetrating Oil Finishes

When it comes to finishing, most woodworkers don’t like to take chances. A guaranteed good result is often the main, sometimes the only, requirement. This is one reason why wipe-on, penetrating oil finishes are so popular. They’re easy and nearly foolproof to apply, offer good protection, and the soft sheen and color of the thin “in-thewood” film is hard to beat.

THE DIFFERENCE.

The attractive appearance of a penetrating oil finish stems from the nature of the film it forms. Unlike varnish, lacquer, or shellac, a penetrating oil doesn’t lay on the surface to create a thick, built-up film. When applied, it seals the wood by soaking in and forming a very thin film at the surface, as illustrated in the drawing at right. The texture and figure of the wood is not obscured and the natural color is enhanced, as is pretty apparent in the photo above. Another benefit is that tinted penetrating oils allow you to apply stain and finish in one step.

SO WHAT IS IT? There’s a fair amount of confusion about the makeup of penetrating oils. Often called Danish oils, they form a distinct category of finish. Although there’s no standard formulation, the recipe is pretty basic. A penetrating oil is nothing more than a mixture of a drying oil (such as tung or linseed oil), a varnish, and mineral spirits. In finishing jargon, they’re termed an oil/varnish mix.

Each of the ingredients plays a pretty specific role. The mineral spirits thins the mixture to increase penetration, the oil adds color and slows the drying time to make the application easier, and the varnish adds protection and durability.

HOW DO YOU KNOW? Most woodworkers simply know a penetrating oil finish by a particular brand name — Minwax Antique Oil or Watco Danish Oil for example (photo above). But beyond relying on past experience and reputation, there is often no way to tell what’s in a particular “oil” finish. The terms oil, tung oil, and wipeon are used extensively on labels. But only some of these products are a true oil/varnish blend.

However, there is an easy way to tell what’s in your finish. You can perform a simple visual drying test. Pour a small puddle of finish on a smooth, non-porous surface, like a sheet of glass, then let it dry and observe the result. The photos below show the clear distinction.

MAKE YOUR OWN. A good way to be certain you’re applying a penetrating oil is to make you own. I like a mixture of one part polyurethane varnish, one part mineral spirits, and one-half part boiled linseed oil. You can vary the recipe to suit your needs. A greater proportion of oil to varnish will ease the application, but decrease the durability.

USING A PENETRATING OIL. In essence, if you can rustle up a clean rag or two, the application is a “nobrainer.” You simply wipe on enough finish to thoroughly soak the surface, let it penetrate for five or ten minutes, and then wipe off any excess. A second coat can usually be applied in six to eight hours. It’s not really possible to build up a thick film with a penetrating oil and this isn’t the goal. So applying more than three coats generally isn’t necessary.

SANDING. The penetrating nature of the finish and the thin film left behind means that the surface needs to be as smooth as possible. The finish won’t fill in blemishes or hide rough sanding scratches. So when I’m planning on applying a penetrating oil, I always finish sand to at least 220-grit. And you want to make certain all the surfaces are sanded consistently.

BLEEDBACK. Open-grained woods, such as oak, walnut, or mahogany, tend to soak up more finish. This can lead to an annoyance referred to as bleedback. As the solvent in the mixture evaporates from the pores, it pulls out the oil and varnish leaving shiny scales on the surface. If these go unnoticed and are allowed to dry and harden, they can be a pain to remove.

The simple solution is to keep watch for the first couple hours after applying the finish and wipe away the spots when they appear. Once the initial coat has been applied, the pores will be sealed and bleedback shouldn’t recur with the subsequent coats.

REPAIRABILITY. The thin film left by a penetrating oil can be a liability and an advantage. The finish may not hold up well under heavy use, but if it’s scratched, dinged, or simply worn, it can easily be repaired or renewed. First go over the area with fine sandpaper or steel wool. Remove the dust and wipe on a fresh coat of finish. The original sheen will be restored and at a minimum, the new finish will help disguise any damage.

Offering all this, the attraction for woodworkers is pretty apparent. Once you try it, don’t be surprised if penetrating oil finds a permanent spot on your finishing roster.