Shellac Under Oil or Varnish

      Finishers discuss compatibility and wear issues with using shellac in combination with Tung oil and varnish. October 13, 2008

Question
I'm in the process of building a maple drafting table for a client and I'm playing around with some finish choices. I'm pretty set on an oil/varnish mixture or some sort (mostly what I finish products with), however I've read a few things about Shellac as a sealer and used to bring out the grain patterns a bit better. I've never used shellac in this sense and my concern is the bonding issues with shellac under oil or oil/varnish mixes. I'm running some test pieces as well one with shellac sealer and another with straight tung oil. The shellac piece really pops the grain. Anybody have any experience in this finish? If so some insight would be nice.

Forum Responses
(Finishing Forum)
From contributor K:
I wouldn't. I think you are asking for trouble with the oil/shellac.



From contributor D:
We use shellac under varnish/oil all the time for our restoration work, and have had no problems with adhesion. Just be sure to use de-waxed shellac (like Seal Coat- or make your own). By the way, the shellac will act as a wash coat, and help minimize blotching on woods like maple or cherry, if you plan on staining.


From Paul Snyder, forum technical advisor:
You can apply oil-base varnish (non-poly) over shellac without any problem. And it doesn't matter if the shellac contains wax or not. The combination of a coat of shellac followed by a few coats of oil-base varnish will produce an attractive and durable natural finish. If you're not using spray finishes and have lots of time for the finish to dry and cure, it's an alternative to the products professional finishers commonly use.

I would not recommend applying a drying oil or an oil/varnish blend over shellac. Drying oils like linseed and tung are a lot less durable than varnish and are not film forming finishes. By adding oil to varnish you only reduce its durability. When you want a "close to the wood" natural finish without a lot of protection from wear, heat, water, etc. then a drying oil or an oil/varnish blend is a good choice.



From the original questioner:
Thanks, your responses have certainly given me some insight to this topic. My finish schedule I had planned was actually as I applied more coats of varnish/oil/thinner to reduce the oil and finish up with just straight varnish. This process is also new to me, but I want to have a nice lustrous finish and hopefully get some film forming as well. I may experiment with some shellac/varnish schedules.

Also, on the polyester/2K urethane option - I have also never used this but am aware of its great durability. My only concern with this is that my table top is two feet of solid maple. With polyester's rigidity, I'm worried the wood movement will crack the stiff polyester.



From Paul Snyder, forum technical advisor:
Forget the oil and stick with varnish. Varnish is made by cooking drying oils with resins. So you get the look of the oil with a lot more durability.

Some varnishes are lighter in color than others because of the drying oil that's used to make them (e.g., soya oil). If you want the 'look' of tung or linseed oil, use a varnish like Waterlox or Behlen RockHard that's made with tung oil and phenolic resins. Durability wise, this formulation is better than an alkyd varnish but not quite as tough as a uralkyd varnish (polyurethane).

You don't want to apply a harder finish (varnish) over a softer finish (oil/varnish blend). That can lead to crazing and cracking. Bob Flexner's book "Understanding Wood Finishing" covers all this stuff in detail. Well worth the trip to your local library or bookstore.



From contributor A:
Paul, does shellac make that much of a difference under the varnish? I would think that a solvent based varnish would "pop" or "wet" the grain enough that the shellac would not be necessary.


From Paul Snyder, forum technical advisor:
I've done some experiments using shellac, drying oils, oil/varnish blends, and varnishes all topcoated with lacquer to maximize depth and clarity. Variations in the final appearance where relatively minor and could be attributed to the amount of color in the finish. For example, linseed oil has a lot of color as well as danish oil, tung oil based varnish, and button or garnet shellac.

Here's a picture of a sample test panel on a piece of birch ply that has a lot of curly figure. Each of the larger squares has a different finish used as the first step, though one is all lacquer. The differences between the squares are subtle.

In French polishing, a drying oil like linseed is used to pop the shimmer/chatoyance and figure in the wood before the shellac is applied. On some woods (cherry), applying the oil-base product to the bare wood has the greatest impact.


Click here for full size image



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