Polyurethane Over Shellac?

Polyurethane Over Shellac? · 10-24-2009, 11:44 PM

Is it possible to put polyurethane as a final protective finish over shellac? I have made two end tables out of some old growth SYP, and the shellac finish really looks nice. However, my wife is afraid that there may be an issue with water rings, so she is wanting me to put on a coat of poly.

Any suggestions or insight?

CarvedTones · 10-24-2009, 11:48 PM

If the shellac is dewaxed, no problem; that's what SealCoat is.

Guy in Paradise · 10-25-2009, 01:43 AM

Sealed wood many times with diluted shellac to provide even staining, then poly or varnish over it. Just ensure that the shellac is fresh and very dry.
I actually like using the orange shellac on mahogany as a sort of stain/tint and sealer, then wipe on poly over it.
I usually put on a couple of coats of shellac, which gives me a thick enough layer to lightly scrub with steel wool, but I must be careful to not scrub through, especially on the corners.
Good luck.

froglips · 10-25-2009, 09:59 AM

Gosh, can't add more to the advise, but.....

It'd be a crying shame to cover up that old wood with plasticy polyurethane!!!

You can wax over shellac, even dewaxed, as the wax will bond to the wax (even the trace amounts) in the shellac.

A few good coats of paste wax and you are set.

With water rings, they are much easier to repair in a shellac/wax finish. They can be removed with denatured alcohol, mayo or a bevy of other techniques for ring removal.

Of course, if this is a case for marital bliss and tranquility, polyurethane is the best idea ever

Jim

Bas · 10-25-2009, 10:52 AM

Originally Posted by rick7938

Any suggestions or insight?

See Howard's advice in a previous thread -> http://www.ncwoodworker.net/forums/f...uestion-26430/

CarvedTones · 10-25-2009, 11:15 AM

To take it a step further, about the only thing I know of that you can't do over dewaxed shellac is something else acohol based that you don't want to mix with the shellac layer.

froglips · 10-25-2009, 11:31 AM

One of the best reasons to never spill one's beer or cocktail

Jim

Originally Posted by CarvedTones

To take it a step further, about the only thing I know of that you can't do over dewaxed shellac is something else acohol based that you don't want to mix with the shellac layer.

Howard Acheson · 11-09-2009, 02:14 PM

Here is something posted by a friend on another forum. You may want to consider its points before deciding on your final finish.

Myths die slowly and are easily resurrected.

No, shellac is not easily damaged. If you have ever mixed shellac from flakes you know that it takes several hours for the flakes to fully dissolve in 200-proof (100% denatured grain alcohol). The alcohol content in gin, even 100-proof gin, is only 50%. In the typical mixed drink the alcohol content is significantly lower. The percent of alcohol in beer and wine is in the mid to low teens. If one spills an alcoholic beverage on a shellac finish and does not then cover the spill by setting the glass on it, the alcohol will evaporate before any damage is done to the finish. Even if the alcohol is captured next to the table by being covered by the glass, and the glass is left there for an extended period of time (i.e. overnight) any minimal damage is easily repaired-far easier than damage to lacquer.

Shellac is among the most moisture resistant finishes available to woodworkers. It does not water-spot!

SteveColes · 11-09-2009, 02:33 PM

Ok, so what about urine & fecal matter. No this is not a joke question. I'm trying to make a potty chair for my grandson in time for thanksgiving. The family will be visiting then.

Howard Acheson · 11-10-2009, 01:54 PM

Originally Posted by SteveColes

Ok, so what about urine & fecal matter. No this is not a joke question. I'm trying to make a potty chair for my grandson in time for thanksgiving. The family will be visiting then.

Long term and/or frequent wetting with bodily fluids will damage shellac. Urine contains ammonia which a powerful solvent for shellac. For a potty chair, the best choice is 4-5 coats of an oil based varnish or oil based poly varnish. Urine damages waterborne finishes.

SteveColes · 11-10-2009, 08:26 PM

Originally Posted by Howard Acheson

Long term and/or frequent wetting with bodily fluids will damage shellac. Urine contains ammonia which a powerful solvent for shellac. For a potty chair, the best choice is 4-5 coats of an oil based varnish or oil based poly varnish. Urine damages waterborne finishes.

Thanks