Installing a larger shower stall, it had backerboard behind it already for the smaller shower with no waterproof sealer, seeing it's a larger size the tile will come over onto the painted drywall. Before installing the tile I was thinking about using Redguard waterproofing sealer on the backerboard and the painted drywall, can you use Redguard on painted drywall for tile?

 

You "can" but I would not advise it... you would not be following the mfg's instructions regarding suitable substrates.

https://images.thdstatic.com/catalog...17eeb4bc38.pdf

While my personal feeling is that drywall has no place in a wet location in the first place, you would be better off using a kerdi membrane if you want to go over drywall.

If its not in a wet location, then it really doesn't need anything in the first place.

 

Assuming the drywall is above the tile, a good coat or two of latex enamel [any sheen] will give the drywall enough moisture protection.

 

What I did was removed the old gray cement board behind the shower, added a couple studs, and replaced it with Hardie Backer Board to the end of the new shower base. The Hardie that is now on the wall is already water proofed and guaranteed by the manufacture, that's why it's a red color. I called the company and was told I didn't have to use Redguard or any other waterproof sealer on their red Hardie product but could do so if I wanted. What I'm worried about is the joint setting compound I had to use to feather my transition from backer board to my drywall, my transition is about an inch past the end of the new shower base. Seeing the backer board wasn't a true 1/2 inch in thickness like the drywall I had to feather it with joint setting compound, there is backer board behind the feather which ran onto the Hardie. Can I just remove the paint and primer down to the joint setting compound and tile over that, or do I have to put a waterproof sealer over my feather that is on the backer board.

 

So this is a new product, Hardie Backer Board with HydroDefence, a waterproof CBU!

Per their instructions:

• When complete waterproofing is required, seal all joints, edges and fastener penetrations with a liquid waterproofing membrane. Follow the liquid waterproofing membrane manufacturer application instructions.

Having tile transition from CBU to drywall outside of the "wet" area is common, how far you need to go is the question.

The fact that you still have to treat the joints would probely just keep me using Prova/Ditra and be done with any concerns!

 

I guess I can just use the Redguard on the backer board to seal all my thinset joints and screws up to the painted setting compound area. I'll sand the paint down up to my transition and try to find something that'll seal the setting compound I can tile over, there has to something out there that'll seal compound.

 

There shouldn't be any joint compound under tile that will get wet, if it's a dry or mostly dry area most any primer will be sufficient.

 

XSleeper suggested using a Kerdi membrane which can go over drywall, I only need to go over about 12 inches of that painted compound on the backer board, I'll check into that also..

 

I see what everyone means now, even after my feather on the transition the backer board is still going to run on a angle into the corner seeing it's only 7/8". Gonna try removing the backer board, then run my 1/4" stripping on my studs to bring it out the 1/2" that I need, hopefully I can save it. Should have just went with Durock cement board which is a true 1/2 inch in thickness. When I put on the Hardie I thought even with the board being only 7/8" I could just build up the thinset behind my tile thicker to make up for the difference.