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Witch-Finder of Lilburn
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I bit the bullet and bought a gallon of spar urethane so that my future mache props might not end up like mush when the rain comes. But what about my other props that are already painted with acrylics? Would it be a bad idea to coat them now with spar urethane?
 

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It should work... just rough-up the surface a little. Be sure you want to do this, though, because you won't be able to paint over the urethane later. Even though all urethanes will yellow over time, it won't likely be noticeable unless you're covering something with a very light color. Dark stuf=no problem.
 

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The problem with the spar urethane is that it's soft & flexible... works great as a top coat, so it can flex with the material underneath. If you paint on top of it, it'll still expand & contract with heat and cold. It will wrinkle the paint on top, enough that it'll eventually flake-off.
 

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You can use regular polyurethane, just not the spar variety. The regular stuff is much harder and doesn't react with temperature changes so much.

We like spar urethane for outside use because it's flexible and can shrink and grow with the temperature swings and not crack. Conversely, the interior stuff, when used outdoors, will crack and allow moisture penetration to the wood.

Paint, even the latex stuff, is relatively brittle, and will crack when applied over such a "fluid" base like spar urethane.

Since you're painting, you're better-off using the kind you'd use on hardwood floors or indoor furniture. Daily, repeated temperature swings and exposure to the elements won't be a problem, so the regular stuff should work great--use lots of thin coats!
 
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