I need to paint a shower in a boxing gym. The shower has all interior cement block walls. I believe the previous owner had just used waterproof paint which is now peeling. Do I need to put up some kind of barrier before I repaint it or should I just prep the wall and get as much old paint off as possible? If so, what type of paint should I use for a previously painted cement block shower?

To prep I would use a pressure washer to clean off the maximum amount of the pealing paint as possible.

Once that is done find a prep for the areas effected,others might chime in as to what to use for that, I use acid,but wouldn't suggest that safely to others.

Primer for the walls and paint with waterproofing 'block fill' paint would most likely be my solution..

I just saw something like this on TV. They used a waterproof caulk on all the joints and a special paint for the block walls to water proof them. It took a couple of coats.