Dewaxed Shellac Fact or Fiction?
By Joseph Fusco
As many of you may know, there is always this "debate" that goes on about using or not using shellac as an undercoating for polyurethanes and now most recently, waterbornes. It seems that the natural wax contained in the shellac is cause for concern to some. Their belief is that this wax will inhabit polyurethanes from "bonding" well to the applied surface. I've always found the topic an interesting one mainly because I've always used shellac (non-dewaxed) was a sealer barrier coat and general undercoat for polyurethane finishes and nowadays I use it more and more with waterborne finish too. I have never had a finish failure. . .
To me there is a very small risk associated with using straight shellac and that the need to either dewax or purchase dewaxed shellac is sulfurous. If one wanted to make there own, it takes nothing more then some time. Allowing your shellac to rest for a period of time will cause the wax to settle to the bottom of the container. You could then "decant" the shellac located above the wax at the bottom.
The one thing I have noticed about using dewaxed shellac is that its
sanding qualities diminish greatly. Like I've said; "there's nothing
like sanding shellac." The problem being that it's the wax that gives
it its great sanding abilities. That being said I'd like to present some
information I've obtained from doing a simple test with straight shellac
and polyurethane.
Recently I did a small piece on blotchy stain where I used shellac as a sealer coat to prevent blotches. I applied one coat of a 2# cut of shellac and varying coats of stain over pine.
This test board was sprayed with two coats of Minwax spray polyurethane
(it was what I had laying around at the time). I have since let the
finished product cure for about three weeks and will use it to do the
adhesion test on. There is no reason to let a perfectly good test board go
to waste.
The only thing that may cause one to raise an eyebrow here is that the
stain was applied over the shellac and then the polyurethane was applied
over the stain. The ideal test would have been to apply another coat of
shellac over the stain once it had dried. All in all I think that this
will sever the purpose. In the future I'll do something more involved.
The first thing I did was to put three types of tape across the surface of
the work piece. Masking, Blue and Duct tape.
I then pressed them down firmly using a padded wooden block. I allowed the
tape to remain on the work piece for about ten minutes before removing
them. The condition of each piece of tape can be seen below. There is no
visible "residue" remaining on either side of the strips of tape
after they were removed .
After that I scored the test board in the belief that it would allow the finish to release more easily from the surface. I scored the board in two different directions. First perpendicular to the grain and then at an approximate 45° angle to the grain
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