Polyurethane comes in multiple forms: interior, exterior (spar), and "poly-shades" (pre colored polyurethane). By its' nature it is designed for interior use only, with no exposure to sunlight. Over the years the formula has been modified slightly to be a little "softer". Polyurethane in its usual form is a very hard finish. As wood moves, the polyurethane "shell" doesn't, therefore causing small cracks in the finish. Not that these cracks are not visible at first. Over the course of years the cracks will enlarge and the finish will literally peel off. Using it on a model is not really a concern unless your model is exposed to sunlight which will increase the speed of the finish failure. One other thing that can speed the demise of your finish is the combination of sunlight and humidity change. (Think model sitting in a window). The UV exposure will speed the breakdown of the polyurethane and the direct sunlight exposure will cause the model to expand and contract unevenly.http://www.nauticalhome.com/
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