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128 - Shellac Under Polyurethane?

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Blog entry by thewoodwhisperer posted 09-25-2010 06:27 PM 8177 reads 3 times favorited 14 comments Add to Favorites Watch

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.

UPDATE 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! I’ll be leaving them outside in the hot Arizona sun to see if there are any differences in the long term.

Products used in this episode:

Zinsser Bullseye® SealCoat™Zinsser Bullseye® 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® SealCoat™

Bulls Eye® ShellacBulls Eye® 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® Shellac

Minwax® Fast-Drying PolyurethaneMinwax® Fast-Drying Polyurethane
Among the most durable of protective coatings, Minwax® Fast-Drying Polyurethane offers long-lasting beauty on both finished and unfinished wood.

Minwax® Fast-Drying Polyurethane

-- For free video tutorials and other cool woodworking stuff, check out http://www.TheWoodWhisperer.com



14 comments so far

View Steven H's profile

Steven H

1114 posts in 1718 days


#1 posted 09-25-2010 07:09 PM

Hey Marc, What Purdy brush were you using?

View Steven H's profile

Steven H

1114 posts in 1718 days


#2 posted 09-25-2010 07:13 PM

I have not tried this, but
Maybe if you wait couple of months eventually you see peeling.

View thewoodwhisperer's profile

thewoodwhisperer

601 posts in 2842 days


#3 posted 09-25-2010 07:17 PM

I just used the best one I could find at Home Depot Steven.

-- For free video tutorials and other cool woodworking stuff, check out http://www.TheWoodWhisperer.com

View PurpLev's profile

PurpLev

8476 posts in 2307 days


#4 posted 09-25-2010 08:14 PM

I think the concerns about such issues are more regarding long term then the couple of days/weeks following the application. it would be interesting to keep test boards for a year or 2 and then see if you can spot any differences. when the finishes had time to fully fully cure and harden, and air and moisture circulation was introduced in and out of the mixture

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View JuniorJoiner's profile

JuniorJoiner

446 posts in 2098 days


#5 posted 09-25-2010 08:15 PM

the finishing method i use regularly is 5 coats of dewaxed shellac followed by two coats of poly. I sand after every coat. no problems.
when you do have problems is using s heavy cut of waxed shellac flake(mixed yourself) and put poly over top. then mild exposure to a heater or sunlight will quickly deteriorate your finish.

-- Junior -Quality is never an accident-it is the reward for the effort involved.

View Stevinmarin's profile

Stevinmarin

837 posts in 1734 days


#6 posted 09-25-2010 10:04 PM

Interesting. You go Marc! Stick it to the man!

Seriously, I have come to suspect that most recommendations like these are geared toward the highest common denominator. In other words, they have tested extreme situations in lab conditions. For 99% of people using such techniques, it will be fine. But the manufacturers are legally covering their asses…just in case.

-- Entertainment for mere mortal woodworkers. http://www.WoodworkingForMereMortals.com

View TheGravedigger's profile

TheGravedigger

963 posts in 2682 days


#7 posted 09-25-2010 11:35 PM

I’d be curious to see a comparison between shellac mixed from flakes and Zinsser’s products. They always seem to behave a little differently. Perhaps it’s whatever they add to increase the shelf life?

-- Robert - Visit my woodworking blog: http://littlegoodpieces.wordpress.com

View tinnman65's profile

tinnman65

1131 posts in 2072 days


#8 posted 09-26-2010 01:11 AM

Great video Marc, my only question is why would you take the chance of having problems down the road on a project you spent countless hours building to save a day of time to let a first coat(seal coat) of poly dry. You may never have a problem, but if you do you have no option but to sand the finish if one arises. Thanks for posting another great video!

-- Paul--- Creativity is allowing yourself to make mistakes. Art is knowing which ones to keep. — Scott Adams

View thewoodwhisperer's profile

thewoodwhisperer

601 posts in 2842 days


#9 posted 09-26-2010 01:32 AM

Well personally I probably wouldn’t take the chance. Generally, I only have de-waxed shellac in my shop anyway. I had to go buy the waxed stuff just to do this video. I just see this claim made as if its a well-known and accepted fact but I don’t see anyone actually experiencing any real problems. So instead of continuing to pass on the info, I just want to see for myself. Finishing is one of those areas that I am just naturally suspicious of advice being passed down blindly. Lots of wive’s tales to work though. So this was just my way of doing a test for myself. I am very interested to see if the finishes degrade differently over time. A month or two in the AZ sun should give us some insight.

-- For free video tutorials and other cool woodworking stuff, check out http://www.TheWoodWhisperer.com

View Mario's profile

Mario

104 posts in 2055 days


#10 posted 09-26-2010 04:14 AM

Hi Marc, if anything survives a couple months under the Arizona sun, please let us know….....

View rando1's profile

rando1

163 posts in 1582 days


#11 posted 09-26-2010 03:45 PM

Marc thanks so much for the study. Great help for the shop in the near future! I have an extra pyrex cup for you, would you like me to send it?
Keep building up!

-- Randon Riegsecker, crosscutservices.com

View TopamaxSurvivor's profile

TopamaxSurvivor

14754 posts in 2334 days


#12 posted 09-27-2010 05:34 AM

Thx Marc! :-))

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View woodkiller's profile

woodkiller

103 posts in 1597 days


#13 posted 09-27-2010 02:56 PM

For some reason the phrase “mix master Marc” pops in my head. All joking aside another great video!

View thewoodwhisperer's profile

thewoodwhisperer

601 posts in 2842 days


#14 posted 09-27-2010 04:32 PM

Should I wear a track suit and a clock around my neck in the next video? :)

-- For free video tutorials and other cool woodworking stuff, check out http://www.TheWoodWhisperer.com

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