Cast urethane will hold paint just fine if you wash it with dish detergent (no lanolin) or simple green or a citrus cleaner, and warm water, scrub with an old toothbrush. This is to get the mold release off. I wash my roof vents in an ultra-sonic cleaner before I ship them because they are so small and would be a real pain to scrub. The roofs will not fit in the cleaner. This cleaning also gets glue to hold. Many people use ca, I prefer epoxy because it gives me time to get things where I want them. I work slow, so I want a slow glue. An hour or so after the epoxy has set, I trim off the excess with an exacto knife before it gets rock hard.
Steve Wolcott