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Q.I have heard that silicon caulk should not be used with cedar trim. Is this true? If so, why?

A.Silicon caulk is not the best choice to use with any wood trim. It works best in sealing the joint between two pieces of non-porous material, such as ceramic tile, metal, or glass. The oil in cedar will reduce the adhesion of silicon caulk, which should be applied to clean, dry surfaces. An acrylic or urethane-based caulk works best where one of the materials being sealed is wood.

How the caulk is applied is even more critical than the type of caulk used. The caulk should be applied so it bonds to two faces across a gap. That way, movement of the materials will place the bead in tension or compression, rather than peel the caulk away from one of the surfaces. A three-sided fillet of caulk in a corner will almost always fail.