128 - Shellac Under Polyurethane

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Original post on our site with additional information, plans, questions & comments:

I have heard countless times that you should never put polyurethane over waxed shellac. From books to magazine articles to forums to DVD's, the message is always the same. Even the back of the shellac can itself says not to use polyurethane. Now I have always taken the "better safe than sorry" route, simply avoiding regular waxed shellac. But there have been so many occasions where I have heard of people accidentally using waxed shellac under polyurethane with no detrimental effects. And frankly, I have never heard a first hand account of a terrible finishing disaster using this combination of supposedly incompatible finishes. So what's the deal?

To answer that question, I decided to do a little experiment for myself. I wanted to see if I could find any evidence of a weakened bond between polyurethane and waxed shellac, when the shellac is used in the typical manner as a sealer. My test is simple and completely non-scientific. There are just too many variables at play to answer this question with any real degree of certainty. But my results gave me enough confidence to say that if you are using the finish as a sealer coat (2lb cut or less), I see no reason not to use whatever shellac you have on hand, even if it has wax in it.

This is a topic that I will continue to watch. And hopefully we'll hear from some folks who have had experiences, good and bad, with this finish combination.

**EDIT** It was immediately suggested that I do a Scotch Tape lift test. I still had the samples in the shop so I jumped in and did a few more tests. Using both duct tape and Scotch tape over a grid work of slices made with an X-acto knife, no lifting of the finish was observed on any of the boards. I even put tape over the area where the epoxy drops were and no lifting was observed there either. These finishes are holding on for dear life!

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[lessnyder]

I understand that I am 12 years late to the party but having been a lab tech in a films company I appreciate this type of trial. One thing you could add to the trial is whats called a "cross-hatch" test which is quite aggressive but should tell immediately if there is going to be a failure. In the lab we use a cross hatch tool but you can use a razor knife and create a series of cross hatch cuts, cover them with different types of tape, allow different dwell times and then peel the tape off comparing how much finish peels off.

[johnathonallen]

If you still have those pieces lying around maybe 12 years later would be a good time to come back to it to see how well they fared.

[shaistafenwick]

Thanks! I’m a baby woodworker and was alarmed at the hyperbolic naysaying about shellac under polyurethane. Thanks for your advice and exploration of the options!

[ccccen]

Had the nicest day today went for supplies at HD and didn't have enough, had to put the shellac  back. This gentleman came up to me and gave me the shellac I had put back as a gift. I want to say thank you very much if he's from around here.

[ttttenney]

I used 1 coat of Shellac under 3 topcoats of Polyurethane on my workbench and it has held up perfectly under lots of use for close to 2 years now. I did that based on this video and have never regretted it.
Thanks!

[pablogazmuri]

In the woodturning world Shellac is consider "universal finish" meaning that anything can go under or over Shellac. That has been my experience. I hope you keep this boards for at least year and give us an update. I enjoy your videos very much, thank you.

[BowserBooBoo]

I have put multiple coats of Amber shellac on 6 panel pine doors and followed up with 2 coats of poly. While I haven't tried pulling the finish off, I can tell you after several years, the doors look wonderful. I used shellac to color the doors because that is the wood finish throughout the house.

[robertphillips]

Mark, Thank you for this video and thank you to all the other viewers that left constructive comments. Both are greatly appreciated.

[woodwhisperer]

Try a natural brush for oil-based poly. Keep the synthetics for water-based materials. The natural bristles will allow you to lay a smoother layer of finish. You might also try diluting a little with mineral spirits to help the finish flow a little better.

[woodwhisperer]

@Shannariano Haven't done any impact testing. But I did do further adhesion testing using a Scotch tape test. I used an x-acto knife to create a grid of cuts and then put the tape over top of that. Neither duct tape or scotch tape could pull up any finish.

[woodwhisperer]

@ironlionkalo Shellac pretty much binds to everything. That's what makes it such a great sealer. It can seal off impurities and oils in the wood, allowing your other more sensitive finishes to bind.

[chihuahuaverde]

Great demo!  I appreciate letting us know to use shellac under polyurethane for drying purposes.  Usually I am waiting ten hours to put the second coat on and your first coat over the shellac looked great.

[Thevoiceoverguy]

To support what Mark is showing us. I have done linseed oil, a spit coat of waxed shellac and 2 coats of water based poly all in one day with no issue what so ever - so yes it can be done!

[tedspens]

The real test would be to expose the samples to heat and cold, like setting them in a sunny window for a while then setting them in the path of an air conditioner. The expansion and contraction might then cause the poly to lose it's bond to the waxed shellac more easily than it would the other samples. I'm thinking along the line of old house trim which has been polyurethaned over and has alligatored over the years. The original oil varnish finish is slightly gummy compared to the more brittle polyurethane, so the poly cracks up and floats around like sheets of ice on water. Of course, the old house trim would have a considerably thicker layer of finish than your samples, but just a theory I feel worth sharing. 

[robw]

I recently saw your other video about shellac and stain vs gel stain. This info is so useful. I'm planning to apply stain to some 100 year old pine and poplar repurposed from an old piece of furniture. Thanks very much for your videos!

[guguyoyo]

The one thing I could think could cause some problems having shellac under the poly would be in a very specific situation. First we would need to have a thicker layer of the shellac under the poly and second long exposure to temperature differences, given that the shellac layer is thick enough to expand so different from the poly that would eventually peel, but than again any other type of varnish over shellac would do the same thing... man this is the reason I watch your channel all the time, very clever thinking. Thanks for the video!

[woodwhisperer]

If you want a shellac finish, you can certainly use that over the stain. I wouldn't recommend staining after the shellac application unless you cut the shellac by quite a bit first. Shellac will seal the surface and prevent the stain from absorbing as much as it would in raw wood.

[blounsb]

I totally get what you mean by testing results for the way you would use it. Just to add to the mix, I did floor sanding/refinishing for years and do wood turning now. We could tell if any wax had hit the floor (even overspray from a can of Pledge) because the urethane would wrinkle immediately. But I use the regular shellac, cut, as one of my sealers for turning without any issues.

[anthonymarker]

12 years late. But, there IS a difference between waxed and non-waxed shellac when you're applying stain (as opposed to poly). You get a much more even coverage of stain with the non-waxed shellac. I've done with this with cherry a number of times. In each case, I seem to get more even coverage of the stain with the NON-waxed shellac, then I cover the stain with a top coat. In my case, Arm-R-Seal. Thanks for the tips, Mark. You got me started down this wonderful road. Much appreciated.

[music-xrco]

recently finished a partscaster guitar...used bullseye amber shellac on a maple body to match the color of the neck..the neck was a gloss finish the shellac was not the same gloss..which i wanted..so rather than start over i used the hand rubbed gloss poly over the shellac..it turned out great and 2 months later no negative signs...quick tip..for a yellow/honey/gold finish..i gave the guitar a yellow stain first..then the amber shellac..it turns out great the way the yellow underneath shows thru the amber..thanks for this video..i'd never have tried it until i saw this..