Hello everyone,
I recently finished building a very large Mahogany entertainment center. I sealed all the wood with Shellac and put on several coats of semi-gloss polly-urethane, after letting the polly cure for several weeks I started to wet sand everything, As I started thru the grits I noticed I was getting severe witness lines all over. almost like a bad jig saw puzzle. Some spots were satin and some were shiny.
I think that the problem is that the shellac wasnt supposed to be used under polly……( I read the can after the fact)
Do I have to start from scratch vand sand everything down to wood? What should I use on top of the sanded wood if I have to sand down again?
Replies
No finishing expert here but
it is a very common problem as far as the >Some spots were satin and some were shiny.< The minerals in the finish that cause the "Satin" settle out and require frequent stirring to keep them "up". It is best to use gloss and then create your own semi by sanding or steel wooling the top of the top coat. Steel wool should even out the finish. as far as the >bad jig saw puzzle< lines . . . heck if I know. I probably would have let the finish dry longer between coats and only used like two or three coats.
Too many coats too quick and the poly underneath won't cure for a long time. You might want to leave it alone for a month and try again.
^^^ What he said.
Plus, could be that you are sanding through the poly and seeing witness lines at the shellac/poly interface.
The warning about shellac under poly is mostly to do with waxy shellac, like Zinsser's Bull's Eye Shellac. You can use dewaxed shellac under poly - and pretty much anything else - without a problem. If you did have a shellac issue, it would most likely manifest itself by the poly flaking or peeling off after a period of time, not while sanding.
You should not use a shellac under a poly varnish unless you use a dewaxed shellac. Urethanes have poor adhesion and will not adhere well to a shellac that contains its natural wax. As Norm says, "always read and follow the directions on the label". They are there for a reason.
What is likely happening is that as you sand you are getting down close to the shellac and the remaining poly varnish film is coming off as a single sheet rather than sanding dust.
There is no way to recover without removing all the finish and starting over. Use a chemical paint stripper containing methylene chloride. Ask yourself why you applied the shellac in the first place. All clear oil based poly or non-poly finsihes are self sealing and a separate seal coat of shellac is not necessary. If it were me, I would just thin the first coat of poly 25-30% with mineral spirits. Thin subsequent coats about 10% to promote smoother flow out. Sand the surface with 320 paper after allowing the first coat to dry at least 24 hour--48 hours is better. Sand in the direction of the grain with the paper mounted on a flat felt or rubber faced sanding block. Vacuum off the dust and apply a couple of more coats thinned as above. Recoat within 8-10 hours and you will not have to sand between coats and witness lines will not be present when you finish the finish. Always give an oil based finish 3-4 weeks to fully cure before attempting to rub out the finish.
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