I have a drywall kitchen backsplash that will be tiled with 3″x12″ subway tile. One area has a concave bow about 3′ across and 3/8″ deep at the worst area. Is it acceptable to fill in the bow with USG Setting type drywall compound, two coats of Kilz2 primer, then tile?
The quick answer to your question is that I would not recommend filling the low spot with drywall patch/compound. I recommend using a rapid-setting patch material designed specifically for this purpose. All of the major manufacturers of tile setting materials have at least one patch material designed for filling low spots on a vertical application for setting tile. Please check with the website of your setting material manufacturer to look for their substrate prep/wall prep/patch materials designed specifically for tile. A void as deep as 3/8″ may require more than one “lift,” but these materials are designed to cure rapidly so as to not take a lot of time, and to allow for the setting of tile to continue without a long delay for cure time.