A.Finish carpenter Gary Katz of Reseda, Calif., responds: There are crown moldings that are meant for exterior use, but MDF is not one of them, and neither is MDX, counter to the widely accepted belief. Your best material choices for exterior crown moldings are either wood or high-density polyurethane. If you use wood, I'd suggest redwood, cedar, Douglas fir, or yellow pine. However, I find that polyurethane moldings are superior to second-growth wood moldings because they hold paint much longer and never warp, twist, cup, or crack. If the material is installed properly, the joints remain tight indefinitely. Several companies manufacture a variety of crown moldings in polyurethane; I'd suggest trying Fypon (800/955-5748, www.fypon.com), Focal Point (800/662-5550, www.focalpointap.com), Chemcrest (800/665-6653, www.chemcrest.com), or Style-Mark (800/446-3040, www.style-mark.com).

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