I would like to finish some pine windows with shellac, but I read that it is “not resistant to alkaline compounds. Alkaline chemicals such as lye and ammonia discolors and mars shellac because of its acidic composition.”
Would it be OK to us shellace and finish with poly over top??
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interior or exterior?
My interior trim is shellac'd without poly over it (gag). No problems, except when the cats barf on the bay window stool. But the nice thing about shellac is that it is easy to repair (just once in the last 4 years), which comes out to 4,592 barfs/coat of shellac. that's a lot of acid. (Actually, acid hardens shellac).
If exterior, use varnish, not shellac.
OK! Another plus for shellac. It's barf-proof! Rich
>> Would it be OK to us shellace and finish with poly over top??
To answer your question, yes, applying poly over shellac is fine as long as the shellac is dewaxed. The wax in standard shellac will not provide good, long term adhesion with oil based poly or any waterborne acrylic finish. Why do you want to first use shellac? Varnish directly applied to wood is just fine.
That said, poly varnish exposed to sunlight will have a very limited life. UV rapidly attacks poly and it will quickly begin to crack and craze. Best to use a non-poly marine finish if sunlight is intense.
Howie,Thanks for your help. I appreciate you sharing your knowledge.I'm trying to match the as close as possible the original finish on the woodwork in the house. The windows are in an addition. My time is limited and I like that shellac as it is easy to apply and dries fast. The windows are on the South side of the house. They get sun in the winter, but are mostly shaded most of the day in the summer.
>>To answer your question, yes, applying poly over shellac is fine as long as the shellac is dewaxed. The wax in standard shellac will not provide good, long term adhesion with oil based poly or any waterborne acrylic finish. Why do you want to first use shellac? Varnish directly applied to wood is just fine.
That said, poly varnish exposed to sunlight will have a very limited life. UV rapidly attacks poly and it will quickly begin to crack and craze. Best to use a non-poly marine finish if sunlight is intense.
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This is true. Most of the work that restoration shops do is not for exterior application. Here are a few tips that will give you a better longer lasting job. Colors are derived from to main sources, mineral and organic. All mineral derived colors (earth tones like black, browns, and tans) are mostly UV color fast. Organic derived colors ( like reds, yellows and oranges) will fade, some quickly, when exposed to UV light. Keep this in mind when your mixing any colors that will get direct sunlight. Urethanes and Spar varnishes are booth good water proof finishes that can be used for outdoors applications. However Urethanes are very clear and act as a glass magnifying the sunlight and UV rays. Spar varnish has a dark amber tint that absorbs UV light.
Steve Nearman
Fredericksburg, VA
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The Master's Touch
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I had a vey old house a while back with windows finished in shellac. The finish was completely alligatored, I expect from exposure to condensation and sun. I love shellac for furniture finishes, but I think I'd go with marine varnish for windows.
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