Should the backer board be in front of or behind the bathtub?
The surrounding bathroom surfaces are adjoined by a bathtub's rim.The tiled surfaces around the bathtub are easy to clean and provide superior water resistance.Some bathtubs have surrounds.To prevent water from entering the wall space behind the drywall, you must seal the edges.If you don't, the mold will grow.If you have a mold problem in the wall, you might have to rip out the drywall.If you want to avoid this, fill large gaps with compound and seal the joint with caulk.
If there are large gaps between the drywall and the flange, fill them with joint compound.Push in enough to make the surfaces flush.It takes no more than 24 hours for the compound to turn white, indicating it is dry.
Use a 6-inch drywall knife to apply a band of joint compound to the edges of the drywall.
Put paper drywall tape over the bands of compound.Joint compound should be used to cover the area underneath the tape.
The knife can be used to wipe the tape flat.To avoid scratching the bathtub, work slowly.
After the tape dries, apply a coat of joint compound.Don't apply so much joint compound that the surface becomes wavy or pitted.The same method is used to allow the compound to dry.