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Polyurethane Not Drying on Cocobolo
Issue: Issue 223
Posted Date: 5/19/2009
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I laminated several hard woods and found that the polyurethane
finish dries on all except the cocobolo. Why, and how can I fix that?
John Brock:
Cocobolo is a member of the Dalbergia genus, specifically Dalbergia
retusa. Dalbergias produce a compound in the heartwood that has the odd
effect of preventing reactive finishes like polyurethane from curing
and forming a hard film. Only evaporative finishes will cure on them,
and not finishes that cure by oxygen polymerization. Evaporative
finishes include wax, shellac, and lacquer. I use a lot of African
blackwood (Dalbergia melanoxylon) and prefer fresh shellac with a top
coat of wax.
Michael Dresdner: As John said,
oxygen cross-linking finishes, which include all oil-based finishes,
polyester and many UV-cured coatings, will not cure on a dalbergia. The
good news is that, once you seal the wood with one coat of evaporative
sealer, you can then switch to whatever finish you prefer. I like to
use Zinsser SealCoat, a clear primer for that job, since it is
compatible under all oil- and water-based finishes and will seal in the
antioxidant that causes the problem in just one coat.