# Zissner Shellac says not to apply poly on-top



## dcutter (Mar 27, 2009)

Hi all - I recently finished building a plant stand that I'm now finishing and had heard many great things about shellac, so I thought I'd give it a try. I've applied a couple of coats (sanded between each coat) and want to apply a polyurethane on top to provide extra protection against potential water spills. The instructions on the Zissners specifically says not to apply a poly on-top. Does anybody have any suggestions as to what I should do? I'd like a bit more protection as I'm under the impression that shellac isn't the most durable finish. Your insight and comments are always much appreciated.

Thank you,
David


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## JJohnston (May 22, 2009)

Poly won't stick because the Zinsser isn't dewaxed. You can apply any varnish that isn't poly - Waterlox, for example (it's phenolic/tung oil).


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## dcutter (Mar 27, 2009)

JJohnston - can you expand on that a bit? I'm not an expert on finishing. Why would a tung oil work on waxed shellac while a poly won't? Will that give me the durable finish I'm looking for? I've used pure tung oil in the past and don't believe that will provide a durable finish. Thanks


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## bill1352 (Nov 8, 2009)

It is not a pretty thing when you do apply poly over it…lol. I did it a few weeks ago. who reads directions anyways. fortunately a heat gun and a hand scraper removed the mess.


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## JJohnston (May 22, 2009)

All varnishes are made from two base ingredients - oil and resin. There are different kinds of each ingredient (Oils: tung, soy, linseed. Resins: phenolic, urethane, alkyd). Each varnish is simply one combination of these two (plus some cooking and other processing). Different manufacturers choose different combinations for different reasons (some are cheaper; some are more water resistant; some are clearer and don't yellow as much over time). Not all varnishes are "polys", but all "polys" are varnishes.

The cheapest to manufacture is urethane resin and linseed oil. This is the DIY "poly" like Minwax. And, for whatever reason of chemistry, a varnish made with urethane resin simply won't stick to a waxed shellac undercoat.

Now, when you talk about pure tung oil, you may be right about it not being durable, but Waterlox isn't tung oil. It's MADE FROM tung oil, but the oil and the resin have been chemically altered somehow to become varnish. It's not tung oil any more, just as a cake isn't eggs any more. And Waterlox is well known as one of the most durable and water-resistant varnishes - perfect for a plant stand. It's quite a bit more expensive than DIY poly, but you can't beat it.

And I'm no finishing expert, either. I hate finishing. But I'm also irritated that finish manufacturers can get away with the degree of deception and obfuscation they do.


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## dcutter (Mar 27, 2009)

JJohnston - That's the answer I was looking for! I will give Waterlox a shot and certainly appreciate your insight to this matter!

Thank you very much!
David


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## BOB67CAM (Dec 28, 2009)

just a side note, zinnser also makes a sanding sealer that is wax free


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## hObOmOnk (Feb 6, 2007)

JJohnston:

*And, for whatever reason of chemistry, a varnish made with urethane resin simply won't stick to a waxed shellac undercoat.*

Can you cite a reference on this?


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## jockmike2 (Oct 10, 2006)

Dang JJohnston I wish I had known that last fall. I had shellaced a bench and when I set it outside it blistered to beat the band, everyone told me to strip it and put spar varnish on it, which I did. It was a lot of work I could have been saved from.


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## reggiek (Jun 12, 2009)

You can also get the wax out of the zinnser by pouring some in a jar and letting it sit for a couple days…and then you can pour off the pure shellac and leave out the wax as it congeals and drops to the bottom (there are some instructions on this in the finishing room in Woodsmith #187.


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## JJohnston (May 22, 2009)

"Can you cite a reference on this?"

Yes - Zinsser's instructions!


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## hObOmOnk (Feb 6, 2007)

JJjohnston:

I'm not trying to be difficult, just accurate.

There is an extraordinary amount of misinformation about wood finishing products floating around the internet.
The more the misinformation gets repeated, the more it becomes *InterFact*.

Here's what the Zinsser Bulls-Eye Shellac instructions say (from their technical department):

"Bulls Eye Shellac is not recommended under polyurethane
finishes. To seal wood before finishing with polyurethane use
Bulls Eye® SealCoat™ Universal Sanding Sealer."

It does not say that urethane resins are a special case and will never adhere to waxed shellac for chemical reasons particular to polyurethane. Nor does it detail what other types of finishes might not adhere to waxed shellac. Furthermore, it doesn't differentiate between oil-based urethanes and water-borne urethanes. Still, it is useful advice.

In fact, many oil-based polyurethane varnishes will stick very well to waxed shellac, especially when there is enough solvent to dissolve and incorporate any wax into the varnish. Chemically the solvent carrier of the oil-based varnish distributes the wax into its mixture and the polymerization process incorporate the small amount of wax into its cured matrix.

This has been proven both in the lab and anecdotally through many years of experience by many professional wood finishers. I followed the regimen of applying oil-based "poly" over un-dewaxed shellac for more than thirty years with no problems. I know many other professional finishers that have the same experience.

Note: Polyurethane or "poly" is another confusing term for wood finishing products. There is no such thing as a standardized "poly" formulation. Most "poly" products vary greatly in their ingredients and production techniques. The actual amount of "poly" can vary greatly between products. I am not aware of any commercial "poly" that uses only polyurethane as its resin. Other resins, such as alkyds, are usually incorporated 
in commercial varnish manufacturing. Example - I use a high-solids varnish that contains eight times as much poly as a popular consumer brand. This doesn't mean that more poly is better. Its the total composition of the varnish and suitability for its intended use that matters.

Just because some of the WaterLox products that are tung oil/phenolic resin varnishes (NOTE: Some WaterLox products do contain urethane) does not mean that they are better suited as a top coat over waxed shellac . This is an extrapolation of the previously misinterpreted chemistry.

*HOWEVER*, I completely agree that *DE-WAXED* shellac makes for a better neutral foundation base for top coats. Zinsser SealCoat is a fine product that I both use and recommend.

If it were my project, I'd strip-off the shellac with sanding and alcohol, then apply a foundation layer of de-waxed shellac

I am neither nitpicking nor trying to be difficult.

Manufacturers' marketing hype and InterFact aren't necessarily science.

Furthermore, I remain open to any fact-based discussion on this topic.


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## JJohnston (May 22, 2009)

At any rate, thanks for answering the original poster's question.


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## SST (Nov 30, 2006)

I, too, have used poly over regular waxed shellac after scuffing the shellac. I've not had any adhesion problems. I even recall reading in Fine woodworking mag that, even tho most folks don't seem to agree, you can use varnish over waxed shellac. That's why I did it that way.

I do agree that Zinnser's seal coat is a great product, and I've used that as well. -SST


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## North40 (Oct 17, 2007)

I wonder … couldn't David put a coat of dewaxed shellac over the waxy shellac and then apply poly over that?


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## dcutter (Mar 27, 2009)

yes - have a couple ideas: either apply a coat of de-waxed shellac or just test an area with straight poly on the current waxed sehllac and see what happens…..

will report back tomorrow.


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## jimc (Mar 6, 2008)

I note one other thing from the original post, in addition to the poly over un-dewaxed shellac issue. David, you mentioned that you waxed between coats of shellac, Unlike varnishes, a second or subsequent coats of shellac do not need sanding between coats for adhesion purposes. Shellacs - there is a term for it but like a lot of other things lately, I can't recall what it is - melt the original coat when a second coat is applied, thus ensuring good adhesion. You also don't need to sand to remove brush strokes because a good second coat will take care of that problem, too.

I recall the information above from a book I read on finishing but as I do a search for shellac sanding between coats, I find dozens of entries stating that you should sand between coats. So take my suggestions above for what they are worth - advice is free; value is ???

Jim


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## hObOmOnk (Feb 6, 2007)

Jim:

Shellac is an *Evaporative Finish*.
Shellac is dissolved in alcohol. When the alcohol evaporates a dry layer of protective shellac remains. If you put alcohol on a dried shellac finish, the shellac will redissolve. Subsequent coats of shellac will melt in to the previous coat.

The reason I sand between coats of shellac is not to create a scuffed surface for bonding, but to level each surface of the shellac to improve the appearance of the final finish. Even when I plan on putting a top coat of varnish, I'll gently sand the shellac foundation layer to the desired quality - I usually use 600 grit sandpaper. That way the final top coat will lay on a well prepared surface.

Varnishes are *Reactive Finishes*. 
They go through a non-reversible chemical process of curing (polymerization). The cured finish won't redissolve in its original solvent.

There are other type of finishes, based upon the chemistry of their components, e.g. catalyzed, coalescing, etc.


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## MyFathersSon (Apr 30, 2009)

I took chemistry in High School as an 8am class with an instructor who was also a school bus driver.
You can do your own math on how seriously that course was taken and how much learned 

When I first starting doing wood work many years ago-I pretty much stuck with Minwax or Varathane with an occassional foray into Deft during the decoupage craze of the 70s. No well thought out reason - they were just what I knew-and all I knew.

As I have read more and learned over the years just how chemically complex and involved the finer and/or more exotic finishing methods can be---- I have learned to have GREAT RESPECT for those who use them. 
Especially those who go back to the old ways of mixing and cooking up their own blends.

Fortunately-for my projects - off the shelf Minwax and following label directions has been sufficient - so far.
Because trying to wrap my brain around discussions like this--makes my head spin like using Kilz in a closed bathroom.
But - I DO find it fascinating - and educational-and who knows-one day I may---- no, probably not.


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## Sawdust4Blood (Feb 16, 2010)

One other issue with Zinsser shellacs to consider. All shellacs begin to undergo esterification once they are dissolved in alcohol. This is why shellac has a shelf life of about 6 months once it is mixed. Zinsser claims their special formula can last up to 3 years but my experience has been that you are throwing the dice anytime the date on the can is more than a year old. IMHO you are always better off buying shellac flakes and mixing them up yourself as you need it. This will also give you a greater selection of waxed and de-waxed shellacs in assorted shades from ultra pale to garnet.


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## Ingjr (Feb 21, 2010)

Funny this topic should come up. I just "used-up" some shellac I had sitting in a can for about 6 months on a shop jig. Planned to put poly over the top until, you guessed it, I read the instructions. After I applied it of course. Had to buy a can of non-poly varnish to use over the top. Whether I had to or not I don't know. Just didn't want to take the chance of refinishing it. Only thing worse than finishing is refinishing. LOL


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## dcutter (Mar 27, 2009)

Went to Home Depot last night - they had neither Waterlox or Zissner's Sanding Sealer so I ended up just using some pure tung oil that I cut down by 50% with paint thinner. First coat went on without any problems. I figure I'll do 2-3 more coats of tung oil and then may put on a final 2 coats of poly to seal against water stains.

One question that I have for everyone - why use shellac in the first place if you are going to put a different finish on top? Is it to stimply bring out the grain in the wood, or does it make applying coats of other finish easier to apply? I understand its use as a sealer against, but apart from that are there any benefits?

thanks again for all the help.

David


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## BillWhite (Jul 23, 2007)

I would not use poly on ANYTHING else.
David, shellac (not Seal Coat) wil make a super indoor, low use, dry environment finish. Seal coat is primarily used as a sealer (duh,Seal Coat-sealer).
Use Waterlox or a wiping varnish on your project and you'll be pleased. Welcome to the world of finishing where everybody has an opinion.
Bill


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## Sawdust4Blood (Feb 16, 2010)

David, there are a lot of reasons that I like shellac. I use it as a sealer after applying dyes, or in a very thin cut before applying stains to prevent blotching, and to help fill grain for a really smooth finish. It is easy to apply and repair. It's so safe it is used as a coating for some pills so they'll dissolve slower in your stomach. It dries very quickly (the fresher it is, the faster it dries) so you can do several coats in one day. It sticks to anything so you can use it to prevent tarnishing on brass. And if you really want to work at it, you can use shellac for the ultimate mirror finish using a french polish technique.


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