128 – Shellac Under Polyurethane

Video - September 21, 2010

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!

Products used in this episode:

Zinsser Bullseye(R) SealCoat? Zinsser Bullseye(R) SealCoat?
This is the only sanding sealer you will ever need! It gives depth and beauty to wood grain, and won?t raise or swell the wood grain. 100% wax-free formula dries in minutes and sands easily. Zinsser Bullseye(R) SealCoat?
Bulls Eye(R) Shellac Bulls Eye(R) Shellac
Trusted by generations of woodworkers, shellac is a traditional, quick-drying finish that is ideal for antiques and fine furniture. It is durable, non-yellowing and imparts a warm glow to all wood surfaces. Bulls Eye(R) Shellac
Minwax(R) Fast-Drying Polyurethane Minwax(R) Fast-Drying Polyurethane
Among the most durable of protective coatings, Minwax(R) Fast-Drying Polyurethane offers long-lasting beauty on both finished and unfinished wood. Minwax(R) Fast-Drying Polyurethane
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