The Wood Whisperer » polyurethane

128 – Shellac Under Polyurethane

thewoodwhisperer — Tue, 21 Sep 2010 18:28:33 +0000

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?
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
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
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

107 – Oil-Based Finish Basics

thewoodwhisperer — Tue, 01 Dec 2009 22:53:03 +0000

Oil-based finishes are typically the first type of finish we confront as woodworkers, be it straight oil or a can polyurethane. Although they are all derived from oil, these finishes can vary widely in terms of application method, durability, and maintenance. The key to understanding these finishes is to understand their ingredients. With that foundation in your tool belt, you can start looking at ingredients lists instead of brand names and labels, and you’ll know exactly what to expect from the finish. Since this is a live session, we have a bunch of viewer questions in the video as well.

Stinky Dresser!

thewoodwhisperer — Thu, 20 Dec 2007 17:42:42 +0000

This week’s second question comes from Melanie. She writes:

My Father in Law made a gorgeous dresser for our son…problem is when we use it, the clothes smell like finish and then so does our baby. We’ve aired it out numerous times and it also aired in his shop before it came over. I’ve tried a couple of odor discs and sachets, but the smell comes back. I’m afraid to try anything stronger for fear of adding/combining the smell. FIL and my grandfather (who also does wood items) are both stumped. What can I use to safely conquer the smell and use the dresser?!

Hey Melanie. This is actually a very common problem. As oil cures, it maintains an odor. Fortunately this odor dissipates over time. But on the inside of a piece of furniture, where very little air exchange takes place, the oil can produce an odor for many many years. So what can you do? The first option, which I’m sure is not feasible, would be to let the unit air out for a very long time. The second option would be to coat the interior with a new finish that doesn’t produce an odor. There are two finishes that are perfect for this: shellac and water-based polyurethane. One or two coats on the interior should do the trick. This material will lock in the oil odor while producing no smell of its own. Your FIL or Grandfather should be able to help you apply either material. So as an FYI for your woodworking relatives, its a good rule of thumb to leave the interior of casework (especially those intended for clothing or blankets) unfinished, or finished with a few coats of shellac or water based polyurethane. That should take care of it.