Finishing Oily Woods
Article - March 27, 2011
This article about finishing oily woods was inspired by a question from Schuyler. He writes:
I put two coats of General Finishes Seal-a-Cell on a piece of cocobolo, then a coat of Arm-r-Seal Urethane top coat (gloss). Three days later, it?s still slightly tacky. Will this ever cure enough that I can continue applying coats of Arm-r-Seal? What gives?
What a Mess!
Many exotic species, like cocobolo, have a certain amount of natural-occurring oils in them. When you coat them with an oil-based finish, the oil in the wood itself prevents the finish from curing properly. The end result is typically a gooey sticky mess.
Fortunately, we do have options. First, you can use an evaporative finish, such as shellac or lacquer. These finishes don’t cure via oxidation and will usually have no problem curing on an oily wood. A second option would be to seal the natural oils in using something like dewaxed shellac. A 1-2 lb cut should do the trick. Start by freshly sanding the surface and remove the excess oil by wiping the surface down with either acetone, lacquer thinner, or denatured alcohol. Immediately follow up by wiping or brushing a light coat of shellac onto the surface. Once dry, sand lightly with 320, vacuum the dust, and proceed with your oil-based top coats.
Now keep in mind, its probably best at this stage to stick with straight varnish and avoid oil/varnish blends such as Danish Oil. An oil/varnish blend works best when it can absorb deeply into the grain, and the shellac sealer coat prevents this from happening. So if you insist on using a blend, be sure to apply very thin coats and wipe off all the excess. Or play it safe and just use a straight varnish that contains no raw oil.
How to Fix It
Now if you are in a situation like Schuyler here, and you have a sticky mess on your hands, here’s what I recommend. The first thing you can do is simply wait. In most cases, the finish will eventually cure. It can take weeks or even months, but it should get there. For most folks, this is not a reasonable time-frame. So another option would be to rub the surface down with some steel wool and mineral spirits. Buff the surface rather aggressively as you attempt to remove some of the uncured finish. Let this dry overnight and see if the tackiness is still present. If it feels fairly dry to the touch, give it a light coat of shellac as described above. If its still tacky, move on to option three: the do-over! Sometimes, the best course of action is to scrape or strip the old finish off, and start over again. It’s not my favorite solution, but sometimes you just have to do it.
Have you had experience with oily woods? How did you handle it?