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What is he best urethane to use on plexiglass?
I am curious. Just why do you want to coat Plexiglass with a urethane? Plexiglass is quite durable all by itself, although it does scratch somewhat easily.
Be advised that all oil based urethanes have a slight amber hue which will make clear Plexiglass look discolored. There are waterborne urethanes which dry crystal clear. There are also waterborne acrylic varnishes. Minwax makes one. These might be the better choice as both Plexiglass and solid acrylic are acrylics, i.e. made from acrylic acid, so the odds of compatibility is better.
I must admit that I am somewhat over my head here. I am not a coatings chemist, but a humble ex-painting contractor. What ever you decide to use, if anything, I would make a sample before committing the whole project and screwing up badly.
Correction: Both Plexiglass and acrylic varnishes are acrylics, i.e. made from acrylic acid, so the odds of compatibilty are better.