What to use to fill gap between green wall board and cement board, and between cement boards around bath tub?
modified thin set. Let dry for 12 hours.
It won't look right doing it your way. You need to add another layer of cement board or take cement board off and fur framing out 1/2 . Watch out if this is on wall with tub faucets as your trim ring for valves may be out too far after you add 1/2. Do all sides so they match.
Use the same material you are going to use to set the tile. Thin set or mastic. They are both water proof when cured. You can pre fill the joints and then come behind to set tile. We would use use fiberglass tape made for drywall and cover that with thin set mortar. Make sure you don't set the green board all the way down on the tub. Leave it up about 1/2 or so and fill with the mastic or thin set so no water can ever wick up into the drywall if grout or caulk fails.
hi any flexible filler, but not silicon as this will collect the moisture and create mould
