I recently set Ditra-heat membrane on my bathroom floor. I then realized I used the wrong mortar type; it’s an OSB subfloor, so I was supposed to use modified mortar whereas I continued following the instructions I had for the Kerdi wall membrane and used unmodified mortar. So I guess the concern with unmodified on wood is that the wood can wick away the moisture from the mortar and prevent it from curing properly. As it turns out, because I had been using the bathroom prior to finishing it, the OSB floor is actually painted. While this is actually good for the curing prospects of the unmodified mortar, I am told that you’re not supposed to set Ditra and mortar of any kind on a painted floor, as it can be a “bond breaker”.

I initially resigned myself to pulling up all the membrane and removing both the mortar and the paint from the subfloor (about 80 square feet). The first piece I pull up was maybe a 5 square foot area. It took me about an hour to get the mortar off. It seemed to be stuck pretty damn well. So then I began to wonder, how likely is it really to cause a problem? Do I spend 40 hours removing all the mortar and sanding the paint off the OSB (or installing plywood/cement board over the paint), or should I roll the dice and press onward? Really leaning toward the latter. Anyone have any anecdotes for tiling over a painted floor?