Michael Dresdner

straight talk about wood finishing

Auntie Oxidant

Q: I made some replacement handgun grips of cocobolo. Realizing that cocobolo has a lot of natural oils in it I first sealed it with shellac, then filled the pores with a product called z-poxy. I then sanded and applied one coat of Minwax polyurethane. Three weeks later I can make a thumb print in the finish. I think I sanded through my seal coat, because the poly is not curing. Can I remove the uncured poly with mineral spirits?
A: I doubt it, unless it is still fully liquid. What typically happens is that exposure to the raw cocobolo (where you sanded through) will inhibit the cure of an oil based finish, but not keep it entirely liquid. Thus, it stays tacky or gummy.
You’re certainly welcome to try scrubbing off the finish with mineral spirits on a coarse nylon abrasive pad, but in most cases you will have to resort to at least refinisher if not stripper. Since the parts are so small that should not be a big deal. You can also sand back to raw wood, again, since the parts are so small.
For the record, the reason oil based finishes don’t cure over dalbergias, such as cocobolo, has nothing to do with the wood appearing oily. In fact, it contains resins, but not, strictly speaking, oils. The problem is that the wood also contains a natural anti-oxidant.
Think of how citric acid, another natural plant anti-oxidant, slows the oxidation and browning of freshly cut apples if they are dipped in lemon juice. The anti-oxidant in dalbergia woods does the same thing.
Oil based polyurethane and varnish cure by oxidation polymerization; they take oxygen from the air and use it to link up small molecules to make large ones, thus turning a liquid finish into a solid film through chemical linking. The anti-oxidant is the culprit in delaying or preventing that chemical reaction.