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Zissner Shellac says not to apply poly on-top

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Forum topic by David posted 04-15-2010 02:10 PM 6413 views 3 times favorited 23 replies Add to Favorites Watch
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David

110 posts in 2102 days


04-15-2010 02:10 PM

Topic tags/keywords: shellac polyurethane

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

-- dcutter


23 replies so far

View JJohnston's profile

JJohnston

1602 posts in 2046 days


#1 posted 04-15-2010 02:13 PM

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).

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

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David

110 posts in 2102 days


#2 posted 04-15-2010 02:23 PM

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

-- dcutter

View bill1352's profile

bill1352

130 posts in 1876 days


#3 posted 04-15-2010 02:29 PM

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.

-- Keep Your Stick On The Ice

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JJohnston

1602 posts in 2046 days


#4 posted 04-15-2010 04:38 PM

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.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

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David

110 posts in 2102 days


#5 posted 04-15-2010 04:47 PM

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

-- dcutter

View BOB67CAM's profile

BOB67CAM

269 posts in 1826 days


#6 posted 04-15-2010 05:23 PM

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

-- if you dont have it, build it, especially when its a stupid idea

View hObOmOnk's profile

hObOmOnk

1381 posts in 2882 days


#7 posted 04-15-2010 08:08 PM

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?

-- 温故知新

View jockmike2's profile

jockmike2

10635 posts in 3001 days


#8 posted 04-15-2010 08:32 PM

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.

-- (You just have to please the man in the Mirror) Mike from Michigan -

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reggiek

2240 posts in 2024 days


#9 posted 04-15-2010 08:33 PM

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.

-- Woodworking.....My small slice of heaven!

View JJohnston's profile

JJohnston

1602 posts in 2046 days


#10 posted 04-15-2010 08:33 PM

“Can you cite a reference on this?”

Yes – Zinsser’s instructions!

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

View hObOmOnk's profile

hObOmOnk

1381 posts in 2882 days


#11 posted 04-15-2010 09:48 PM

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.

-- 温故知新

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JJohnston

1602 posts in 2046 days


#12 posted 04-15-2010 10:10 PM

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

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

View Shopsmithtom's profile

Shopsmithtom

780 posts in 2949 days


#13 posted 04-15-2010 10:21 PM

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

-- Accuracy is not in your power tool, it's in you

View Peter Oxley's profile

Peter Oxley

1426 posts in 2629 days


#14 posted 04-15-2010 10:24 PM

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

-- http://www.peteroxley.com -- http://north40studios.etsy.com --

View David's profile

David

110 posts in 2102 days


#15 posted 04-15-2010 10:28 PM

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.

-- dcutter

showing 1 through 15 of 23 replies

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