Cocobolo and poly not drying

[sawdustus]

I came across this question, and the answers to it, while reading my latest online issue of Woodworkers Journal. I thought it might be useful to those of you who use cocobolo and related woods. The answers are by two of their resident guru's.

george


Polyurethane Not Drying on Cocobolo
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.

[Janette]

Very interesting! thank you for sharing.

[wood-n-things]

Well,
who would have thunk it.....Thanks for the tips...I still don't know what they are talking about and I eat my coco puffs with milk any way...

[workin for wood]

That's pretty much the facts right there, but the poly will dry. It will possibly take a couple months to do so and by then be a nasty dust infected mess, but it will dry. I typically use acrylic urethanes, which is not on that list, but is an evaporative finish and dries no problem. I always say, if in doubt, drop shellac on it. Shellac is like garlic...cures anything. Shellac can go over or under anything.

[Fremmed]

AHHH.
A friend gave me a couple small pieces of cocobolo that I cut into Christmas ornaments. I sprayed them with urethane and the finish never came out right.
You live, you learn.
Thanks George