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The instructions for Zinsser brand clear shellac says: "not recommended as a sealer under polyuerthane finishes"

I have Minwax Helmsman Spar Urethane. Can a urethane finish be used over shellac?

What is the difference between polyurethane and urethane. The Minwax Spar Urethane can says not for use on floors. For floors use polyuetrhane.
 

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First lets define a few terms.urethane & polyurethane are the same resin used in making a varnish. there are others alkd,phenolic and others. Spar varnish is a long oil varnish originally made to be used on spars of ships. The reason the can says not to use over shellac is because polyurethane varnish has adhesion problems, it doesn't even stick very well to itself. That is why you need to sand each coat so it will adhere,not so with regular varnish. The makers of any polyurethane varnish(it is varnish made with a urethane resin) don't recommend using over natural (contains wax) shellac and neither do the makers of water borne acrylic. Personally I don't believe polyurethane varnish should ever be used on any furniture. If you insist on using that varnish over shellac (why ?) use dewaxed shellac.

Regards
Jerry
 

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Thanks for the input Jerry I've been thinking of using shellac for my next project. I was wondering what the difference was between the two.
 

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If you want to try something different than the prepared shellac off the shelf try some of the lesser refined grades such as garnet,dark brownish red good on walnut,mahogany and darker cherry.Seedlac warm unrefined brown excellent for restored antiques(must be strained) and many others.Shellac is a very under used and under appreciated finish.

Regards

Jerry
 
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