A.Debbie Zimmer, director of communications at Dow Chemical Co.’s Paint Quality Institute in Spring House, Pa., responds: In general, paint performs very well on low-maintenance trim products made from various combinations of PVC, polypropylene, polyethylene, polyurethane, polystyrene, and cellulosic fibers. But some kinds of composite trim contain small amounts of the release agent used during the forming or molding process, and this may interfere with paint adhesion. To avoid problems, you should prep the trim’s surface before painting.

Form-release agents can be either water-based or solvent-based. A quick wipe with mineral spirits or denatured alcohol (followed by appropriate drying) will remove any waxlike residue, while a quick wash with water and detergent will get rid of any water-based residue and dirt. If mildew is present on the trim, remove it with a 3-to-1...

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