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Polyurethane over shellac

Misterp's picture

Just curious. Has anyone applied Ben Moore clear poly over surfaces sealed with shellac (against directions on can)? Am wondering what possible problems might be encountered. Have been unable to contact BM regarding this warning. I have done this once or twice and had no problems.


thanks

douglas2cats's picture

(post #107119, reply #1 of 14)

The shellac warnings are usually there because it wont work over shellac that has wax in it. It should work fine over dewaxed shellac though. As far as I know, the only pre-mixed dewaxed shellac is Behlens SealCoat. So if that's not what you used or you didn't mix your shellac from flakes and know it's dewaxed, it's probably not going to work.

Waddaya mean it wont fit through the door?

If you build it he will come.

jerrymayfield's picture

(post #107119, reply #2 of 14)

I don't use polyurethane varnish,but both alkd and phenolic varnish work fine over shellac natural or de-waxed.

Regards
Jerry

Pondfish's picture

(post #107119, reply #3 of 14)

Poly works fine over shellac, either waxed or de-waxed.  Shellac happens to stick to just about anything.


The "SealCoat" shellac another poster mentioned is from Zinsser, not Behlens.  It's basically a 2lb cut of dewaxed shellac, and very economical versus making your own from flake.


Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
douglas2cats's picture

(post #107119, reply #4 of 14)

Thanks for correcting my momentary memory lapse re: Zinsser. Dont know why I was thinking Behlens.

Waddaya mean it wont fit through the door?

If you build it he will come.

SteveSchoene's picture

(post #107119, reply #5 of 14)

There are plenty of polys, including practically all waterborne polys, that do not adhere well to shellac containing wax.  But even the waterborne polys with adhere to shellac.  Of course I haven't tested many of the waterborne polys so can't be totally dogmatic about it, but since problems could take some time to appear, why risk putting poly of any kind over shellac with wax.  Traditional resin varnishes--alkyd and phenolic do stick over shellac and provide good protection in all but the rarest of situations. 

Test your finish on scrap, FIRST, or risk having to scrap your finish.

Pondfish's picture

(post #107119, reply #6 of 14)

Yes, I too used to believe that you could only apply a finish over dewaxed shellac.  But sometime in this forum stated that this was not true, and he had no problems.  I've since applied poly (albeit not water based) onto shellac (waxed) without any problems.  A Zinsser rep also confirmed that most anything will stick to shellac (which is a naturally sticky substance).


I'd encourage testing out the finish.  Too often, dogma are created that get passed around as fact.  I think the "only used dewaxed shellac" is an example of such dogma.  Another dogma got busted in this month's FWW, where sanding, scraping and planed surfaces were compared for finishing.  They confirmed what I had seen years ago--there is no real difference in the final finish.  But there are some out there who insist that, to get a good finish, you must ______ (fill in with either sand, plane or scrape).


I'm a former scientist and my mantra has always been "do an experiment."  In my experience, the experiment of applying poly onto regular old shellac provides a fine finish.  I'll extend that to see what a water based poly will do, the next time I have a home project where I can see what happens over time to the finish.


 


Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Misterp's picture

(post #107119, reply #7 of 14)

In my initial post I neglected to complete the can warning as saying not to apply over shellac or ANY surface sealed with a sanding sealer. I too have applied poly over both these surfaces and have no problems or customer complaints. Thanks to those replying.

SteveSchoene's picture

(post #107119, reply #8 of 14)

I saw those tests but came away somewhat unimpressed.  Its not the difference you see in a small board that are significant, not the difference in the look under a magnifying glass. I make period reproductions, and the difference between a hand planed finish and one prepared by sanding, particularly with factory sanding by wide belt sander, can be seen across the largest living room.  Eighteen century surfaces were never flat but had the small undulations running with the grain left from planes.  This is particularly noticeable before rougly mid-century.  As the century progressed planing became finer, and scraping reduced the undulations as furniture styles progressed from Queen Anne to Chippendale.  An even bigger change occured with the transition to the styles of the Adam brothers--in the U.S. the Federal Period furniture following Sheraton and Heppelwhite.  Surfaces became flatter still with the use of veneers and the exquisite gloss of French Polishing. 


The hand planed look isn't for all styles of furniture.  Certainly furniture manufactured frankly to express the modern manufactured age doesn't need hand planing.  But to me, I see no point in using the techniques available to amateurs and small commercial furniture makers to try to reproduce the products of factories.  Go to museums and galleries for inspiration, not to Pottery Barn or Ethan Allen. 


Got a bit afield from the topic--sorry. 

Test your finish on scrap, FIRST, or risk having to scrap your finish.

Pondfish's picture

(post #107119, reply #9 of 14)

No problem -- I liked your input about period vs. modern furniture, and how the undulations caused in a handplaned surface are noticeable and typical of period work.  No argument there, and your point is a good one for anyone who looks to extend the results of FWW's report beyond the experimental boundaries. 


My point was meant to say that there has also been dogma put forward where "only a hand planed / scraped / sanded to 220 (or 320) surface gives a good finished result."  This is a dogmatic statement that is not supported by experiment.  I did a test ages ago where I compared 220 sanded, 320 sanded, and scraped surfaces to each other.  I saw no differences between the 320 and scraped, and a slight loss of gloss in the 220 sanded surface.  I don't do period work, so the macroscopic differences are not important in my pieces.


Getting back to the original thread -- do you have direct experience in finish failures after applying poly over waxed shellac?  I'd be interested to hear info like that.  The manufacturer's recommendations are the first ones I take with a big grain of salt, but other's experience (not dogma) is priceless.


Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
SteveSchoene's picture

(post #107119, reply #10 of 14)

I don't directly, but thats because I have read finishing books by people like Bob Flexner, Michael Dresner, and Jeff Jewitt.  And I do remember some of the early days of poly when people would write anquished posts about peeling coats of finish because they had waited three days to put on a second coat without sanding the previous one.  Its true that poly is better now, but my point of view is why take a risk when it is not needed.  I don't see any purpose in using poly for furniture.  It has a use for floor finishes, or for kitchen cabinets if they must, for some reason, have a brushed on finish.   But for furniture I wouldn't pay a dime or take a smidgen of risk to use a finish that is hard to rub out and doesn't look as clear as similar thicknesses of traditional resin varnish.  And I don't go to the extra work of a traditional resin varnish on most furniture since shellac provides a more beautiful finish that has passed the tests of time.  Over a hundred years I'd bet more shellac finished furniture looks good than poly finished. 


Let me add that manufactures don't give "bad advise" out of error, per se, thats not in their interest.  But, they can't recommend thinning finishes because of VOC regulations, and they never (except makers of marine varnish) recommend enough coats because they don't want to appear to be harder to use than the competition.  But when they say not to use over shellac, its almost certainly because they have had complaints, and don't trust consumers to properly distinquish between waxed and dewaxed shellac. 


Edited 10/31/2005 1:11 pm ET by SteveSchoene

Test your finish on scrap, FIRST, or risk having to scrap your finish.

Pondfish's picture

(post #107119, reply #12 of 14)

All good points--but I'm still curious to see if modern poly formulations have problems peeling off of shellac.  In my experience (limited, granted, as I dislike the look of poly as much as anyone), non-water based poly sticks to on-dewaxed shellac and stays on for at least a few months.


I agree with your sentiment about why take the risk?  This is always a good way to go forward, namely, don't risk a problem if you are not sure.  I'm sure that I'm just splitting hairs here, namely, do we really, really know that poly over shellac won't work.  Despite poly being ugly, despite the risks it might not work. 


I also agree that many manufacturer's recommendations are to keep consumers in line with VOC regulations.  But sometimes I find their recommendations just plain self serving (like "only apply blahblah varnish over blahblah brand sealer.")  SO I take them all with some skepticism.


Despite my doubting Thomas attitude, I do appreciate your input and in keeping this thread civilised!


Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
HowardAcheson's picture

(post #107119, reply #11 of 14)

As a scientist, you may want to perform an experiment.  Prepare a surface of maple sanding from 120 through 180 grit.  Now apply a natural (wax containing) shellac to one section and a dewaxed shellac to another.  Let them dry two days.


Apply an oil based poly varnish over a section of each and a waterbourne poly varnish over another section of both.  Let them both dry two weeks to develop full adhesion.


Now use a sharp exacto knife and cross hatch each finish with cuts through the finish every 1/10 of an inch.  Do the same at 90 degrees.


Press down a strip of duct tape--scotch tape will work too--onto the finished area.  Let it set an hour and then remove the tape.  Count the squares of finish that get removed with the tape.  Up to 5% indicates good adhesion.  10% is average and less than 25% is unsatisfactory. 


This is the standard test for finish adhesion. 


I have done the test a number of times and poly varnish over natural shellac has always resulted in 50% or more failure.


Howie.........


Edited 10/31/2005 4:02 pm ET by HowardAcheson

Howie.........
Pondfish's picture

(post #107119, reply #13 of 14)

Thanks -- a good protocol and I'll try it out sometime to see what the results are.  It will be a good project for a rainy weekend when my curiosity is in need of a fix.

Recommending the use of "Hide Signatures" option under "My Preferences" since 2005

Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
SteveSchoene's picture

(post #107119, reply #14 of 14)

Thanks for the hard data.  Have you done the same comparisons for "traditional" resin varnishes? 

Test your finish on scrap, FIRST, or risk having to scrap your finish.