Is it ok to do this? Do you need shellac first?
Also, how about poly over lacquer? Do the same rules apply?
Why would you want to mix finishes? One or the other is good enough. ;o)
Neither is OK. Laquer and poly have very different chemical/physical characteristics (both surface adhesion and expansion/contraction). It would be best not to layer the two - better to strip one or the other off of the piece and re-coat with the final finish.
I agree it is best not to mix finishes unless really needed, such as shellac to seal an oily wood from an oil based varnish or the like. At least he would get away with putting oil based varnish (poly) over lacquer although long term durability might be compromised a bit.
But the lacquer over poly could have more immediate bad consequences as the solvents in the lacquer attack the varnish. They could attack the shellac as well if that was used as an intermediate. (Lacquer thinner often has ethanol as a dilutant.)
Test your finish on scrap, FIRST, or risk having to scrap your finish.
Plus - Shellac can be directly dissolved in Laquer thinner. It's a pretty good way to apply shellac that'll dry a bit more slowly, allowing you to keep a "wet edge" on a big surface for a tabletop.
I've read in a number of sources that poly will go over shellac and stick, but I wouldn't do it. Polyurethane has very different chemical properties than oil-based natural or aliphatic resin varnish - I'd be concerned that the poly would de-laminate from the shellac after a number of years of expansion/contraction due to humidity changes.
Varnish over shellac (dewaxed for poly varnish) is a very common feature of many finishing schedules where the shellac is needed to separate materials. I don't have enough examples in my own work over time to make the case empirically, but many such schedules come from pretty reputable sources so I'd bet the loss of durability is pretty minor. After all, with the shellac nearer to the wood and being the harder, more brittle material, I wouldn't think the more flexible poly would add much in addtional stresses to the shellac. It's rather like following the fat over lean rule artists use for oil painting. Also the shellac will retard moisture transfer more than poly alone, so the total wood movement will be slowed, and likely reduced if it is seasonal.
Of course, the durability problems would be greater if the shellac underneath is a thicker film than what you might get with one or two 1 1/2 lb. cut barrier coats.
It also comes to mind that the single part, consumer grade polyurethane varnishes that is almost universally meant when the term poly is used isn't anything like a pure polyurethane either, the resin is alkyd modified with added polyurethane resin. My limited experience with pure polyurethanes suggests, based on manufacturers instructions for substrate preparation, that they may have more significant adhesion problems if there are substrate problems. But that's a horse of another color entirely.
Edited 6/24/2008 2:04 pm ET by SteveSchoene
Edited 6/24/2008 2:29 pm ET by SteveSchoene
Shellac will not readily dissolve in most lacquer thinners. There is no such thing as a "standard" lacquer thinner as all thinners are proprietary mixtures of solvents and vehicles. If there is a substantial alcohol component engineered into the solvent, it may dissolve a bit of shellac, but such a lacquer solvent is very unusual.
Put a few shellac flakes in a glass jar with most any lacquer thinner. They'll be quite intact days later.
I remember a post here by someone who recommended cleaning a spray gun that had been used for spraying shellac with lacquer thinner because, he contended, it was a superior solvent. He kept insisting that lacquer thinner was the way to go until he actually tried dissolving some flakes in it as I advised.
Then he admitted his surprise and how clean his gun was after he had re-cleaned it with alcohol.
Rich - Laquer thinner is not as good a solvent for shellac as ethanol, of course, but it will dissolve roughly a 1.5 lb. cut. I use it quite frequently, and it's been written up in a couple of different finishing books. To get it to dissolve, the shellac has to be ground (I use a coffee grinder), and it helps if the laquer thinner is slightly warmed by placing it in a warm water bath. Given laquer thinner's high vapor pressure, the container cannot be sealed, of course.
My concern with poly over laquer/shellac is with differential expansion rates of the two films. I would suspect it might take quite a while for the layers to craze or separate, perhaps years, but I try not to make anything that won't outlive me. ;-)
I certainly can't refute your experiences since I don't have the materials you're using.
I always grind my shellac in a coffee grinder.
I've had no success in getting shellac to dissolve to any usable degree in Sherwyn Williams' solvent for their CAB Acrylic lacquer or in generic lacquer thinners from Home Depot (different suppliers at different times. The CAB Acrylic dissolved in both).
Let me answer this way. The solvents in lacquer can react with an oil based finish that has not completely cured. "Completely" should be taken to mean a couple of months. Even then, it's a risk. However, oil based varnish or poly varnish can safely go over lacquer with few problems.
Thanks you for the reply guys.
The reason I asked this question is because I have been doing touch up work on new construction. All new stuff is mostly lacquer based and I have good success with my minor repairs/touch-ups. But occasionally I get a request for a repair from a private individual. I really don't know what the finish is on their table or furniture item. My repair products are fast drying lacquer based products. I just want to provide a quality repair, even if the finish I want to repair is poly. My repair products are designed to dry quickly. Burn-ins and epoxy repairs go quickly but info on compatibility with poly is nebulous.
Thanks again for your replys.
I was taught as a rule of thumb , you can put anything over lacquer but not to put lacquer over anything .
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