Things You'll Need
6 inch drywall knife
Paper drywall tape
A bathtub's flange, or rim, adjoins the surrounding bathroom surfaces. Generally, the surfaces around the bathtub are tiled, which provides superior water resistance, and are easy to clean. But some bathtubs have drywall surrounds. Water easily degrades drywall, so you must seal the edges to prevent moisture from entering the wall space behind the drywall. If you don't, the moisture buildup likely will lead to mold growth. In severe cases, you might have to rip out the drywall to deal with the mold problem within the wall. To avoid this, fill large gaps with joint compound and then seal the joint with silicone caulk.
Fill with joint compound any large gaps where the drywall meets the flange. Push in enough to make the surfaces flush but not so much the joint compound oozes out. Allow the compound time to turn white, indicating it is dry, which typically takes no more than 24 hours.
Apply a band of joint compound roughly 6 inches wide to the drywall edges that meet the flange, using a 6-inch drywall knife.
Lay paper drywall tape over the bands of compound so that it butts up against the flange. The area underneath the tape should be entirely covered by joint compound to ensure proper adhesion.
Wipe the tape flat with the 6-inch drywall knife. Work slowly to avoid scratching the bathtub flange.
Apply a coat of joint compound over the installed tape after it dries. Hide the tape, but don't apply so much joint compound that the surface becomes pitted or wavy. Allow the compound to dry fully before performing another coat, using the same method.
Sand your work with 150-grit sandpaper after the compound dries. Don't scratch the bathtub flange.
Apply a narrow bead of silicone caulk where the drywall edges meet the flange. Use the tip of your finger to smooth the caulk so it is consistent and even. The silicone caulk provides a watertight seal.
Read the caulk manufacturer’s instructions carefully to ensure the caulk is suitable for use in bathrooms and that it can bond with drywall surfaces.
You can use masking tape to keep the bathtub’s flange clean while you work, but don’t let the masking tape get caught underneath the joint compound and caulk otherwise you risk compromising the seal.