Varnish over Shellac?
#11
  
I am building a dining table out of white pine. Knowing that pine can be hard to stain I looked in Fine Woodworking for advice and found an article from 2007 on the Best Finish for Pine and followed the instructions. I first applied a washcoat of thinned Zinsser shellac followed by the oil-based stain followed by another washcoat of shellac. The article says the best top coat is varnish because it adheres well to waxy shellac. It says not to use poly urethane because it will not adhere.
The Rustoleum spar varnish I got says on the can not to apply it over shellac and to make sure there is no wax on the wood.
Which is correct? Can I use the varnish over the shellac? I went ahead and applied the first coat of thinned varnish to the legs but it has not fully dried yet. Before I do the table top I want to make sure it will stick. Let me know if anyone has any advice. Thanks, Tom
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#12
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
(12-28-2018, 08:23 PM)Tom Wiarda Wrote: I am building a dining table out of white pine. Knowing that pine can be hard to stain I looked in Fine Woodworking for advice and found an article from 2007  on the Best Finish for Pine and followed the instructions. I first applied a washcoat of thinned Zinsser shellac followed by the oil-based stain followed by another washcoat of shellac. The article says the best top coat is varnish because it adheres well to waxy shellac. It says not to use poly urethane because it will not adhere.
The  Rustoleum spar varnish I got  says on the can not to apply it over shellac and to make sure there is no wax on the wood.
Which is correct? Can I use the varnish over the shellac? I went ahead and applied the first coat of thinned varnish to the legs but it has not fully dried yet. Before I do the table top I want to make sure it will stick. Let me know if anyone has any advice. Thanks, Tom

Rustoleum spar varnish is a  oil modified urethane resin  (poly urethane).   What Zinsser shellac did you use ? If you used the Seal Coat shellac that is dewaxed you are ok.  The difference is how the varnish is made.   Roly
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#13
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
The problem isn't with the shellac, it's the urethane resins that inhibit adhesion; and the reason for the warnings. It's easier to warn than to explain the difference between waxy and de-waxed shellac. It's also possible you won't have any problems, but best to not take a chance. So, like Roly said: which Zinnser did you use? If it was seal coat, go ahead and do whatever you want. If it wasn't seal coat, you could use a non-poly varnish like SW Fast Dry Oil ( careful, they may make that in both poly and non-poly formulas...read the label) or P&L 38, which is a little hard to find. What most folks call "polyurethane" is actually varnish, it's just made with urethane resins instead of alternatives.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#14
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
Thanks for the information. I thought varnish and poly urethane were two different things and the FWW article seemed to confirm that.
I used regular Zinsser shellac. Can I put a coat of Sealcoat over the regular shellac and then use the varnish I have? I had a hard time finding “varnish” locally so I ordered the Rustoleum online. I know I can get the Sealcoat locally but am not sure I can easily get the proper varnish you recommend.
Thanks again for the suggestions. I have a lot to learn about finishing. Tom
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#15
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
There's mixed opinions about that as well (seal coat over regular shellac). Shellac will dissolve into itself, you will often hear that several coats of shellac are really just one. What happens is that some of the wax may come to the surface when you top coat waxy shellac with seal coat. I don't know what the truth is, no experience doing that. In the words of woodworkers today, many think "polyurethane" is a different finish than "varnish". If oil based, it is not. Varnish is a compound made by cooking a resin with a drying oil. The oil is normally linseed oil, tung oil, or soya oil. The resin is usually polyurethane, but can be alkyd or phenolic (or others less commonly). Once cooked the whole mix changes to a compound called varnish.....it the magic language we speak that has led to everyone using the term "poly".
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#16
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
Thanks. I still have the problem of what to do with my table. I don’t want to mess it up. Can I wipe down the top with alcohol to remove some of the shellac and then topcoat with Sealcoat? Any other suggestions? Tom
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#17
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
My suggestion (if you want a guarantee of no problems) would be to choose a different top coat, one of the 2 I mentioned above. Both are really good products, and there's SW stores everywhere (it seems). I prefer the P&L when I can find it, which is very seldom. You can try removing the shellac, but you are setting yourself up for the possibility of having a bigger mess to deal with, and you still may not get all the wax off. Should you chose to go ahead with the removal, i think I'd try scraping it off first, then sand it smooth and start over.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#18
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
I went to the SW store and found out that they have discontinued this varnish.. They are selling Minwas products now. However they had one can of their varnish left that they sold to me for less than half price - $9. I will let you know how it works. Thanks again for the advice. Tom
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#19
  Re: RE: Varnish over Shellac? by Tom Wiarda (I went to the SW sto...)
FYI, Sealcoat and rattle can shellac both are dewaxed.  All the others made by Zinsser contain wax.  I have often sprayed Sealcoat or rattle can shellac over Amber shellac, for example, before applying WB polyurethane varnish and have never had a bonding problem.  I would do the same if using an OB poly varnish.  

Sealcoat is the universal sealer.  It bonds to most everything, and most everything bonds to it.  

John
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#20
  Re: Varnish over Shellac? by Tom Wiarda (I am building a dini...)
It is an error to think that spar varnish is necessarily better than regular poly.  The spar varnishes and exterior varnishes are softer and more more flexible to deal with the temperature changes  and humidity changes that exist outdoors.

For an indoor table that is not an advantage, and the harder surfaces of standard poly are probably preferred.  

My go-to for interior tables is oil based poly.  I refinished all the tables at the local Starbucks after they got soft from constant cleaning.  I sanded off the finish (too easy) and applied 4 coats of Minwax Oil based poly.  

When they  retired the tables after 10 years the tops looked pristine.  The one caution I took was to wait 2 weeks after the final coat before putting the table tops in service.  

It is doubtful that a home table will see the abuse that the busy restaurant's tables had.  So I am comfortable with this recommendation.
No animals were injured or killed in the production of this post.
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