New guy here. Just did the thin-set mortaring of the seams around my tub. I went past the concrete board on the ends and onto the drywall with the mortar and am now thinking that was careless and is going to look really bad if i try to paint over it as it in no way has the same texture as the drywall or the drywall compound I am using. Duh. What is the best thing to try and sand the mortar off with so I can get back down to the drywall compound underneath (or right to the drywall and compound over it again)?
2) once tile is set and grouted, take a stiff 3" putty knife and make a light pass to remove any loose thinset, then simply skim coat over the excess thinset with joint compound leaving the wall smooth and ready for primer. If the excess thinset has any high points, instead of using sandpaper get yourself a rub brick at Home Depot and carefully grind it down. Don't worry about scuffing the drywall 'cause the skim coating will cover it.
The brick will work fine to knock down any large bumps....I've done this type of thing many times. Since he's skim coating the area with joint compound no need to worry about minor scuffing of the existing DW. Sandpaper is fine for smoothing, but it is the wrong approach for large bumps.