A.I would not consider moisture-resistant (MR) drywall appropriate for use on an open porch. While MR drywall is resistant to deterioration from moisture, it may sag from direct exposure to moisture. MR drywall is designed to be used behind bathroom finishes, but not to be directly exposed to water. The joints and nails in MR drywall are usually left unfinished, since they will be concealed with tile or other finish materials. The joint compound would not withstand the weather exposure on a porch.
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