Urethane over Spar Varnish
From contributor B:
It is a poor practice to put a harder finish over a softer one. You need to trip or sand the spar varnish and then you can put whatever you want on the table. The glass the previous poster recommended is the only exception that comes to mind.
From the original questioner:
The owner wants a finish on it. They didn't want to spend the $200.00 on a glass top. I was thinking about completing the job with a polyurethane finish.
From contributor C:
You've received some good advice. Spar finishes are formulated to remain soft and flexible to accommodate the swelling and shrinking of woods as moisture content changes. This is especially important on exterior products. In addition, nothing likes to stick to urethane - not even urethane - so there needs to be a good mechanical bond between coats. Now we want to mechanically bond soft flexible urethane with hard rigid urethane? You'll end up with finish failures and be right back where you are now. You'll be far ahead if you strip off the soft stuff before you refinish.
From contributor D:
Ditto. Spar is a long-oil varnish, meaning a higher ratio of oil to resin. Regular old polyurethane is a short oil finish meaning it has a higher resin content. Higher resins are what you want for interior stuff - scuff and chemical resistance. High oils are what you want for exterior stuff to allow for wood movement as mentioned above. Bite the bullet and strip it - it shouldn't be terribly difficult. Use paint stripper or maybe even cheap reclaimed lacquer thinner and a good scraper.
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