I have a brick fireplace that has paint on it. I plan to install 12x12 slate or granite tiles on it.
Do I need to strip the paint off first or will the thinset stick to the painted brick?
I would install rib lath over the existing brick, mud in a scratch coat of mortar followed by a brown coat smoothed out and leveled leaving some knotched trowel marks in it to give the thinset something to grip on.
Thank You. How does one attach the lath to the brick?
We always went with fender washers and tapcon screws for masonry attached in the grout lines. You can also run a "j" channel at the base and where the brick meets the wall. It installs first, (at all perimeters), then the rib lath over the back flange. You can mastic on some 30# felt before the rib lath as well as an underlayment, but being indoors, not really neceassary, imo. Here's a site that has lath products to give you an idea. If you have a big box store close, they should have everything you need.
Here is a link that might be useful: lath products
I did just what you're describing, tile over painted brick.
Sierraeast's suggestion is bullet proof. I did something different though. I took as much paint off as possible and scratched the brick surface. I used a wire brush on a drill and more or less banged up the surface with whatever worked. Some mild chiseling. I did this 3 years ago, have fires a lot, and no problems. You might also look into a product that acts to bond thinset to painted brick - I think it exists but I'm not sure what it is. A remodel job I was associated with not long ago had a similar requirement and I was told the concrete guy would use such a product but I never found out what it was. Myabe someone here can verify or dismiss that option.
We had a similar project at our last house. The fireplace was built in 1918, and the mantle was formed by turning and stacking the last 4 or 5 courses of brick. We removed these courses and replaced them with bricks laid at the same depth as the ones below, used a mini grinder to take most of the paint off the rest of the bricks, roughening them at the same time , then applied the thinset and covered with brown italian marble. We bought a wooden mantle that had been removed from a turn of the century house that was being demolished and installed that as well as an insert. Fantastic result and I would send pics but we sold the house