Is there any problem for the mortar to adhere to Redgard painted Hardie backer? Putting large wall tile 8"x8" on tub surround wall. What mortar is better?
There is no problem with thinset adhering to Redgard painted backer. Just make sure there isn't a vapor barrier installed behind the backer board or you'll trap in the moisture. Hopefully, you taped all the seams on the backer board. Use a fortified thinset. Let it set 5 to 10 minutes before applying. Set the tile and wait 24 hours before grouting. Good luck.