Sticky finish problems

We've been having problems getting our Bocote and Cocobolo to finish up nicely. We're using pretty small pieces of wood, around the size of a pen blank after it's been narrowed to a half-inch or less - it's mostly pieces for my wife's hair jewelry, but I'd been hoping to use some dark cocobolo to make the black pieces for a chess set as a gift to my brother also - not as dark as ebony, but much prettier I think.

However, it'd be a bit pointless if I can't make the wood coat nicely, and these two are proving to be a real pain. As was recommended to us before, we're using two coats of dewaxed shellac followed by two to three of oil-based polyurethane (satin or gloss) as the outer coat, sanding lightly between coats. To coat, we alternate between the piece standing vertically (mounted on a small pin, using a hole drilled into the base that will be covered later) and hanging the piece so that it is upside down instead. We need to use a coating that is fairly resilient, and yields a waterproof finish.

But we're finding, sometimes, that even after it seemed to have dried nicely on the previous coat, it then gets sticky with the next. Right now, we have a piece that was almost dry last night, with just one small sticky spot that had not quite cleared up - today, the sticky spot is bigger. It doesn't happen with every single piece, but I'd guess it happens at least 75% of the time.

Is there anything we're likely doing wrong, or anything different that people have used to coat these woods without the coatings turning sticky?

Rob, two coats of dewaxed shellac is good advise but instead of the poly you might look at something like General Finishes Poly / Acrylic blend or even a brushing lacquer (Deft maybe).

I have no explanation as to why the oil poly is sticky other than perhaps it is not thouroughly dry between coats. When you sand it, does it "corn" on the paper? If so, you might want to give it a bit longer. Could also be the oil from the woods interacting with it - although the shellac should seal it in pretty well I would think. I have limited experience with cocobolo and none with bocote so I can't say for sure.

Hope you discover the cause . . . .

L

What kind of a finish are you looking for? High gloss? flat? real woody? Also what lb cut of shellac are you using. If you are looking for a bit of build then I would coat with shellac (1 1/2lb cut or thicker) until you just start to get a build. Then lightly sand and start in with your finish coat. I would consider Arm-R-Seal or lacquer for a top or just continue with the shellac and buff. You might also consider sanding the raw would out to 1200 grit and then try CA as a finish.

I've had the same problem - can't figure it out either.

The shellac should have sealed it. But it didn't. You might need to wait as long as week, maybe even longer for the finish to dry.
Could be dry tomorrow too, one never knows.:mad:

Try wiping with naptha or acetone immediately before applying the first coat of shellac.
Let the shellac dry a day or two before top coating.

And... avoid sanding until two topcoats have been applied. If you burn through the shellac at all oil will bleed out and disturb the topcoat.

Hope this is helpful.

Well, best guess then is that maybe we cut through the shellac coat...that certainly sounds possible. I thought I was being careful, I'll take extra care with that this time. And acetone is probably worth investing in, as cheap as it is - we thought we had some, but can't find it, so we're off to pick up a can of it now. We only just bought a can of General Finishes poly/acrylic blend to try, so we may give that a shot as well. :)

I just finished stripping off the last of the coats from the previous effort, so good or bad, I'll post again when I see how it's all turned out.

Wish me luck!

I've never tried a water based finish over an oily wood so I can't say anything about that with certainty.
Seems as though it should work as long as the seal coat holds the oil at bay. Otherwise I'd think adhesion might be more problematic than with an oil based finish.

Good luck and look forward to hearing of your success!

I haven't worked with oily woods but have seen posts where folks clean the oils off with mineral spirits or the like just before applying the finish. Maybe someone who has done this will chime in.

Mineral spirits isn't quite up to the task.
Acetone and naptha or other strong, fast evaporating solvents seem to work best.

Yeah, we picked up a can of acetone rather than the naptha, and after wiping with that the dewaxed shellac is taking nicely onto the bocote so far. The second coat's already on.

The dewaxed shellac reckons that it should take any coating, so the poly/acrylic should be okay, but we want to be sure we'll like the finish before we commit it over the top. We have another piece testing with the poly/acrylic coat at the moment, and we'll see what we think of its finishing before we try applying it over the top of the shellac.

We may retry the oil-based poly anyway, though, just to see if the change to sanding and the use of acetone improves its curing before we move onto trying the poly/acrylic. I figure it's worth knowing the answer to that one either way... :)

Rob;
Although not identicle to yours, I also had a sticky problem with Cocbolo. I made a birdseye maple table top with breadboard ends in cocobolo. I applied natural danish oil then let dry a couple days. Then I put on Minwax wipe on polyurethane, wiping the cocobolo with acetone first. The maple dried over night no problem, but the cocobolo stayed sticky for several days. I gave up.
I sanded it all smooth to 400, wiped with acetone, then brushed on Deft gloss lacquer. The cocobolo seemed to take the lacquer top coat and dry nicely. After a day I'm going to put on another coat of the lacquer.

The poly never seemed to work over the oily wood.
Good Luck, Mike

Well, so far, it's going well, but tomorrow will be the real test of the final result. The bocote was wiped down with acetone, then two coats of dewaxed shellac and two coats of oil-based polyurethane were applied before I did a light sanding (400, finer than the can recommended, but I figured it might work out better) to smooth it out, and now the third and final coat is on. By tomorrow morning, I should know if it's worked out properly, so fingers crossed! :)

Looks like this has done the trick! We're now the proud owners of one nice piece of bocote - rubbed down with acetone, coated twice with 2lb dewaxed shellac and then three times with oil-based polyurethane, sanding lightly with 400-grit before the final coat. Nice to know the answer's so simple... :D

The best guess, then, is that the sanding I'd been doing - though gentle - was cutting a little too deep. Because it's only been sticky in one spot that'd usually then spread and get worse, I'm thinking that maybe I just caught the dewaxed shellac a little too hard in one spot, so that although the seal was mostly intact, the one spot was broken enough to let the wood's antioxidants do their oily thing and muck up the curing.

Never again, though - that's the last time I listen to the instructions on the can for finishing. Thanks for the help, everyone!