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.

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