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Finishes on Finishes

Recently used chalk, rubbed into wood for coloring; rubbed into wood, then brushed and vacuumed off excess. I then applied one light spray coat of shellac to seal color and then finished with spray polyacrylic. I was afraid to brush on the polyacrylic, fearing the color would run -- all worked out well. I did try a couple of test pieces first. All this brought about the thought/question: how well do various finishes work in combination with others, e.g. lacquer with polyurethane?, etc. Would appreciate any thoughts, experiences...

Thanks, Bill Burlingame

Hi Bill,

Also would be interested to hear responses and suggestions. Coloring and finishing my pieces always causes me to lose sleep and adds a few grey hairs to my head...

There's an article in the current Fine Woodworking magazine on finishing that talks about a few combinations of finish. I've successfully used:

wax over a seal coat of shellac

wipeon polyurethane over oil based stain over a seal coat of shellac

Some sealers will work under either oil or waterbased finishes but not all (read the can or read an online spec sheet first). The wood you use makes a big difference. Wood with wavy grain has to be sealed well before finishing especially if you use stain. Best approach is to try what you want on a scrap of the same wood but even then I've had issues.

Like Mary says it can be a scary business.


Bill, Tim is on the right track.  There definitely is a hierarchy of compatibility that MUST be respected unless you enjoy starting over.  The best and most foolproof way to prevent incompatibility is to stick with the recommendations of a manufacturer.  They can be contacted if there is any doubt.  And to mix things up, as Tim alluded, the density and grain of the wood has to be considered.

There are times, as in my case, where the local suppliers do not carry a full line of anything, ands you mix and match at your own risk.  So some general knowledge is helpful. A general rule of thumb is that finishes based on the same solvent are compatible, e.g. oil base over oil base, lacquer over lacquer, water base over water base.  Those three general categories are incompatible with each other for the most part with very narrow exceptions.

Now for every "rule" there are exceptions.  You can shellac over most anything, but the bond will differ from purely mechanical (poor) to full chemical blend (best). Thinned, it's a great sealer, but you must consider the top coats. Lacquer can be applied to oil stains if they are thoroughly dry, more than 24 hours and still it will pull the stain into the first coat.  Lacquer can be applied over a thin shellac but it's pretty pointless to do so, and it doesn't work to put shellac down to seal the stain and then lacquer.  Thinned lacquer makes a good sealer. Lacquer does well over a water base stain or an aniline dye. Lacquer will wrinkle just about everything else. Varnish is oil based and will work well over any stain or dye, and shellac or thinned lacquer is a good undercoat for it to lock the colors. Varnish can also be used over an oil-varnish mix like Danish oil or Antique Oil.  You can varnish and shellac over lacquer and some water based stuff, but the bond is mechanical. You want the current coat to bond chemically and mechanically with the previous coats, and the only way to ensure this without a degree in chemistry is to stick with what is known to work, i.e. oil on oil, lacquer on lacquer, water over water.

I've saved polyurethane as a separate subject because it is a beast unto itself.  It isn't compatible with anything, even itself. Yes, you can put poly over poly but to get more than a poor mechanical bond you must recoat within 24 hours so the solvent can bite the previous coat. It simply won't bite chemically into any  cured previous coat, so the best you can hope for is a mechanical bond.  It is not considered a serviceable finish, i.e. it is not generally repairable because it cannot be feathered to previous coat. Wipe on polyurethanes leave a super thin coat which can look pretty rich.  Sprayed and brushed poly simply layers and can be difficult to smooth for next coats, partly because the smoothing has to be done while it is still "green", i.e. not fully cured. The wiping polys help with this because they dry smooth. The good news is that polyurethane is impervious to just about anything that might try to damage it.

And if you decide to shuck it all and use a varnish stain, or colored finish coat, good luck.  Sometimes it works, sometimes it doesn't, mostly doesn't.

A paint store specializing in finishes is a good resource because they generally know all about this stuff.



Greg Howlett and Robert Butler have reacted to this post.
Greg HowlettRobert Butler

DanK, as well as all others.

Thanks very much, I'll be making myself a chart from all your tips for reference -- please keep them coming, for all of us.

I'm particularly thankful for the tips that help with the sealing/protection of the chalk coloring, because of the pastel affect it gives.

"...fair evenings and mornings merry." J.RR. Tolkien