29 April 2019 at 9:38 am #557391
I’ve just finished a coffee table for my sister. The top is laminated pine boards, and the finish will be a polyurethane varnish, for durability…
In the past when I’ve used this I’ve always applied it to bare wood…
However since discovering the Sellers way of things I’ve discovered, (amongst other things), shellac sanding sealer, and usually use this before applying a water based varnish or wax finish…
Is it worth applying shellac based sanding sealer before polyurethane varnish or is it a bad idea?
Thanks in advance
Matt30 April 2019 at 3:51 am #557500sanfordParticipant
I wonder why you would want to apply shellac before a poly varnish. I doubt it would harm anything, but would it add anything? It is sometimes used, I think, to seal knots that might leak sap, but I bet, without direct knowledge, that poly will do that too. (I have applied shellac to pine before poly just to see whether it changed the appearance. Just a slight difference in color from the shellac maybe. Perhaps you might want an amber shellac for its color under poly. Hm . . . ) Shellac is fine as a topcoat for some purposes — not usually for a table top that might need more durability and which might get booze spilled on it since alcohol does a number of shellac. Other than that, I have only used it to separate layers of finish. For example, I like oak — though not pine — with added color. So I might use a water based dye with shellac over it to seal, and then a dark glaze before the final top coat. That is pretty common with oak.30 April 2019 at 4:04 am #557504CraigParticipant
Your next call should be to the customer/ technical service department of the manufactirer of the coating you are considering using.
Explain what you’re trying to accomplish and follow their advise.
SW Pennsylvania30 April 2019 at 10:15 am #557532
I’m not actually wanting to do it… just wondering if there is any advantage generally in doing so.
I think I’ll give it a miss. As I said, I’ve never done so before. I’mprobably just overthiking it!
Thanks for responding,
Matt30 April 2019 at 11:10 am #557546Larry GeibParticipant
I think your instinct for shellac is sound unless there will be lots of alcohol around, like a bar top.
Shellac was a standard prep in a boatyard I worked in. It is a better vapor barrier than poly, is harder, and will assure a better bond to the wood. You can also get several coats in a day and sand between coats. Fixing mistakes is also easier.
Some poly won’t stick well to itself once dry, so sanding is a must and the time between coats is longer.
Most poly is tough, but not hard. There are exceptions. Behlen Rock hard and Pratt and Lambert #38 are a couple. Price them out and shellac will sound a lot more attractive for multiple undercoats.
One caution. I’ve always used waxed or I waxed shellac under spar varnish. That works fine, but I believe poly requires unwaxed shellac ( Sealcoat)
And Ive found recoating dried poly doesn’t work well unless you sand first to give the new coat “bite”. If you miss spots, it can delaminate.30 April 2019 at 12:37 pm #557567
Thanks for that…
Unfortunately just too late.
I’ve just given it a first coat of poly varnish!
I’m planning on 3 coats. I’m sure for a coffee table it will be just fine, and I was overthinking things!
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