Hey wood web! I’m completely stumped on an issue I’m having involving various layers of tinted dewaxed shellac and matte oil based polyurethane.

After doing various tests we came up with a finishing regimen that was a very close color match on some new moulded red oak to the existing chestnut trim installed in an early 1900s house in west Philadelphia.

The test boards we did were fairly short boards but all turned out looking great but now that we are applying the regimen on the actual new casing we are getting an inconsistent alligatoring effect! See attached photos.

The regimen:

1. 1 coat of full strength fast drying minwax matte poly applied by foam brush (acting as a wash coat and to prevent the next coat of tinted dewaxed zinsser shellac from penetrating too deeply in the grain. Shellac is tinted with transtint dyes and the tests I did without the first layer of full strength poly beforehand darkened the grain in a way that strayed from the look of the original chestnut we are matching)
2. Allow to cure overnight (at least 12 hours)
3. Quick 400 sand and tac.
4. 1 coat tinted dewaxed zinsser shellac using transtint dyes. Applied by roller and then smoothed out by foam brush.
5. Allow to dry 4 hours.
6. No 400 sand because it was burning through color too easily on the test boards I did…but we did do a quick tac before next step.
7. Another coat of full strength fast drying minwax matte poly applied by foam brush.
8. Allowed to cure over a full weekend (48+ hours) and up until this point everything was looking as expected and normal and felt and looked fully cured.
9. Quick 400 sand and tac.
10. Another 1 coat tinted dewaxed zinsser shellac using transtint dyes applied by roller and then smoothed out by foam brush.
11. And then finally 2 coats of fast drying minwax matte poly.

The test boards I did with this regimen worked and looked perfect but the issue I’m having now with the alligator pattern is showing up between steps 10 and 11 on the trim boards where we get the second coat of tinted shellac on and it starts wrinkling out in an inconsistent way where not all parts or even every piece is developing the alligator pattern but almost all of them and in an inconsistent way.

I’ve considered it’s possible we are applying too heavy of a coat but it happens even when we try to do thinner coats too. We really can’t seem to find a pattern as to what’s causing it.

I’m going to do some more tests now skipping the poly coat between the two shellac coats hoping that may help. My reasoning for doing the poly coat between though was to avoid the second coat causing the previous coat to pull up during the second coat application and I wanted to kind of seal it in place and have had luck doing this with previous projects.

Any insight would be much appreciated!

In the attached photos I’ve shown examples of the alligatoring effect and then one photo of a piece that for some reason did not develop the alligatoring at all.

Thank you so much!

View higher quality, full size image (4032 X 3024)

View higher quality, full size image (4032 X 3024)

View higher quality, full size image (4032 X 3024)

DOnt mix poly and shellac. Apply the shellac by spraying it or apply a dye stain with no shellac in it and it will bond to the shellac.

Pete, can you say more about not mixing poly and shellac? To be clear each coat is cured before the next one goes on and is not mixed all together.

The shellac is dewaxed and the client wants a matte sheen as top coat over the look of the shellac and I’ve personally done this previously with no issues as long as it’s cured.

Transtint dyes will not tint poly and the existing woodwork we are matching has a shellac look and likely has it which is why I went this route. Transtint mixed perfectly into shellac and using it in this way as a toner looks right.

If I apply dye to the raw wood it darkens the grain in a way that strays from matching the original woodwork.

You have a hard layer(poly) with a soft layer(shellac) that is causing the alligatoring. Eliminate the poly and you will be fine. If you need to seal the wood first, use shellac. Then lay up your color on top. I would never foam roll shellac. I also always mix my own from flake. You can pad on the color or spray. Rolling the shellac will pull the color so do it a different way.

Thank you Pete! This all makes sense and I think I will move towards sealing with shellac first instead of the poly but I’m still a little stumped as to why the first layer of shellac on top of the initial polyurethane sealing coat did not cause the alligator effect.

What the real issue here is the solvent from the shellac is attacking the coat of poly... The alcohol in the shellac is essentially acting like you threw stripper on the poly.. that's why it is wrinkling.

Definitely try going with a clear shellac seal coat and then 1 or 2 tinted shellac coats before using the poly. Plus as mentioned above spraying would be the preferred method for the tinted shellac coats as brushing may end up moving the color of a previous coat around the surface.

BH Davis

I've used water based poly over shellac with no issues. You need to let the shellac coats dry completely, more than 4 hours, maybe 12-24 hours or more. The alcohol in the shellac coats needs to fully evaporate out of the finish so it doesn't attack the poly.

I would not recommend mixing finishes like you are doing and other people are suggesting here.

BH- If you put too much shellac under poly, the poly will crack. A thin coat as a sealer will most likely be fine but if the shellac gets built up too much, the poly will crack. I ran into this many years ago and never did it again.

Why are you layering very different finishes? Theres no need to and what you think you may be gaining is just adding more risk of potential failure.

Pete, I was taught out of Bruce Hoadleys book on finishing (and is also my belief from experience), that shellac is a very hard finish, and oil based poly is much softer. Just look at a shellac flake to see how hard it is, and see dried poly on the side of the can to see how malleable it is. While you can mix the two, understand the differences, and make sure every coat has had ample time to cure.

Ive experienced the same failure issues with shellac under poly. I dont see any reason to put shellac under poly unless you need some form of thin barrier coat. But even then, there are much better top coats than poly unless you are finishing a floor....

There are multiple posts on here about using Sealcoat under poly, especially water based poly. Lookup "crazing and Sealcoat" (or "krazing"?).
Some think it has something to do with the pH and say "real" shellac works much better.
Many think a very thin layer of Sealcoat is required.
It seems universal that you need to let it dry much longer than 4 hours.
I just had this on some fir doors that I wanted to have a solvent look under Sayerlack H2O poly. I waited until the next day in all cases to spray over the Sealcoat, but not necessarily 24 hrs. In the areas that I had to overlap my pattern, like the inside edge of the rails, I got some crazing. That tells me that either it was too thick, or because it was thicker, it needed to dry longer. The crazing was minor so I could mostly fill it in with subsequent layers of finish. It did not go away completely though. They are vertical man-doors and in my personal house, so I went with it. Learned a lot though.