Alligatoring with Waterborne Poly over a Shellac Sealcoat

Pros dig into the reasons why water-based poly might show fine cracks and wrinkling over shellac. February 23, 2008

From everything I've been able to find on this forum and other resources, there should be no problem applying a WB polyurethane over Zinsser's Sealcoat. I'm trying to protect a stained and amber shellacked cabinet near a sink from water damage. On sample pieces, I experienced hairline cracks and alligatoring when I applied WB poly (Zar) over either the shellac itself, or the Sealcoat. Any suggestions?

Forum Responses
(Finishing Forum)
From contributor B:
I had the same problem with Fuhr topcoat. I was able to fix the problem by sanding the sealcoat before applying the poly. I think the problem is that your sealcoat is too smooth and therefore those cracks show.

From the original questioner:
I sanded it. Maybe not enough. I'll try again. Thanks.

From contributor A:
Often, when what you are describing occurs, the culprit is too thickly applied coats, or subsequent coats applied over a prior coat which has not dried thoroughly. Many WB products do not like to be applied in thicknesses greater than 2 - 3 wet mils. If the WB coating is applied under cool and/or damp conditions (less than 65 F), each coat can take substantially longer to dry sufficiently. The material will appear dry to the touch, but will be not be sufficiently dry to topcoat.

It's generally not a good idea to use shellac (even as a sealer) in any area that's subject to repeated water contact or in high moisture areas. Topcoating shellac with a water resistant coating provides only limited water resistance, and, if the piece is repeatedly wet, the finish may eventually fail. A better system for use in such areas is a vinyl sealer topcoated with a pre- or post-cat lacquer or conversion varnish - or - a waterbased (WB) system which meets KCMA specifications (will not include shellac). I'm not aware that the Zar product meets KCMA specs for moisture resistance. If the product is listed for exterior use, it should be fine.

From contributor W:
I've used pure dewaxed shellac cut in alcohol with WB finishes without any problems for many years, both interior and exterior, water exposure or not - it has worked well for me and other finishers. If the shellac is fresh and properly cut (dissolved) it will work well in a water exposure environment as long as the topcoat of choice is capable of handling long-term water exposure. The issue you are facing of cracking/crazing has more to do with the SealCoat reaction to a high pH over-coat, i.e. WB's, than it does with water exposure. The SealCoat chemistry is not friendly to moderate/high pH WB's.

I have seen Zinnser SealCoat craze underneath many different WB finishes - Minwax, Fuhr, General, SW, MLC, Target, Bona Kemi and Absolute Coatings (LastnLast) WB floor finishes.... You name it, SealCoat can fail under WB due to the pH and ammonia reactions common within WB chemistries. If you want to ensure a successful shellac application underneath any WB finish I suggest that you use pure dewaxed shellac flake cut in denatured alcohol. It will handle any WB finish when freshly cut and applied in thin, multi-coat applications.

To repair the problem at hand, fine sand the topcoat back to meet the underlying sealer coat. Level sand to remove the craze/crackle defect, then apply 2 coats of fresh dewaxed shellac and overcoat with your WB of choice. This should isolate the problem.

From the original questioner:
Thanks for all the timely replies. Is this place great or what? I will put this info to work. Contributor W, do you have a suggestion as to the ratio of flakes to alcohol (i.e. the cut)? Luckily my crazing problem has occurred on sample blocks, so no need to repair anything.

From contributor M:
I would suggest that the shellac be good and dry before you coat over it, and that you first apply some mist coats over the shellac or sealcoat, before you apply a flow coat of the WB poly. I believe that the SealCoat is a dewaxed shellac, and is a 3 lb cut.

From contributor D:
Shellac is actually a very good moisture barrier, and can effectively seal the wood from water vapor (in the air). It is not so good with actual water on it - but as a sealer with a decent topcoat on it, it is fine.

From contributor A:
What's in the Zinsser Seal Coat chemistry that would make it respond any differently from any other dewaxed shellac to a high pH coating? I thought Seal Coat was just basically dewaxed shellac in alcohol in a 3 lb cut.

I've never experienced any difficulty using it under Fuhr, Target, and Hood WB coatings - both acrylic and urethane. I've never used it under a catalyzed WB, such as Bona Kem. I always apply a thin (1 - 2 mil wet film) first coat of all WB finishes.

From contributor G:
Thanks contributor W - you cleared up a mystery that has been puzzling me for a while.

From contributor R:
You might have better luck using Target's WB shellac. It would not react to a WB top coat.

From contributor I:
I agree with you about shellac being a good moisture barrier. But I think it might actually be the regular waxed shellac that gives it that characteristic? Testing done on sealing lumber on the ends of boards to help minimize splitting showed that good old wax worked the best! Read that somewhere... There seems to be two schools of thought on this subject. I have been using it (Seal Coat) very successfully over oil based stain and then WB poly (Varathane Floor Finish and/or Spar Varnish - both WB). So, if the wax is important as a moisture barrier, should one use regular shellac, aka Bull's Eye by Zinnser? Funny thing is, SealCoat says right on the label that it is "Guaranteed under any finish."

From contributor P:
I have used Sealcoat very successfully under several different manufacturers' WB topcoats without the problems described. I cut the Sealcoat 2 to 1 with denatured alcohol giving me a 1.5lb cut (Sealcoat is a 2lb cut, regular shellac is a 3lb cut), spray on a couple thin coats, let dry overnight, scuff down with 320, blow/wipe off dust, then apply one, maybe two, tack coats of the WB topcoat followed by a really thin wet coat, let dry, scuff, apply normal wet coat, see how it looks and scuff and apply another coat if needed.

I believe the key to success is letting the shellac really dry and then getting some relief in it by sanding with 320 and not putting on a wet coat of WB to start with. I have also used this same technique with WB topcoats over regular amber shellac, and haven't had any complaints/returns.

From the original questioner:
What is a tack coat?

From contributor Y:
A tack coat is a light misting coat of finish you spray on the piece you are working on. I think the idea is to introduce the two disparate finishes. Then when the tack coat is a little dry you go over it with a full finish.

For what it's worth, I've had the problems you describe with various shellacs and various waterbase finishes. The point about the chemistry of the SealCoat causing problems is interesting and should be looked into.

But I basically use contributor P's method above and haven't had many problems. Don't put the shellac on thick, and keep your first w/b coat on the thin side as well. If you keep having the problem, there are other finishes that will work.

From contributor W:
Contributor A, I'm not sure what the full "hidden" chemistry of the SealCoat is, if any. Although it is dewaxed shellac flake in alcohol, they must be doing something to it in order for the clean-up process to work, i.e. cleaning brushes and equipment used for applying SealCoat by rinsing with ammonia and water, instead of cleaning up with pure denatured alcohol. It makes sense if you don't want to buy alcohol for cleaning, but maybe there is something going on with the use of ammonia and how it raises the pH to dissolve the shellac during the cleanup process...? Maybe this is why the SealCoat is crazing? Not sure, and it does not happen all of the time, but I've seen it enough to be wary of it.

From contributor M:
I think when Zinnser first came out with their Sealcoat, it was intended as a ready-for-use coating, and many shops were using it mainly as the sealer for water base coatings because those wb coating were wetting the wood and raising the grain. Others were using it as barrier for silicone, while others where using it for French polishing. It certainly was working fine for many years. There have been many changes in the water base chemistry, and maybe that is why so many finishers seem to be having problems.

From contributor K:
Check out Target's discussion board as there is a lot to say about SealCoat vs H20 shellac.

From contributor A:
You can use ammonia to remove or strip any shellac, not just seal coat. It's much easier to use than trying to use alcohol. You just have to neutralize the raw surface before applying a new finish (white vinegar, followed by water).

From contributor K:
I've used ammonia as a rinsing agent (a cup to a few gallons H20) for years after stripping with meth chloride and never had trouble finishing after that. I do rinse well after it though.