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Installing a shower...put in PermaBase cementboard. Is it a good idea, bad idea, or indifferent....to apply a latex paint primer to the cement board before the tile installation?

FWIW..the sides are cementboard....and the ceiling is MR and I primed the MR...just not sure whether or not to prime the cementboard. I will be putting in tile floor, walls, ceiling.
 

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Primer does 0 to water proof anything. It's porous.
It's made to give a bonding surface for the paint.
 

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I had written more, but somehow it didn't get posted.

RedGard is a fine idea (although I use Kerdi and bypass the use of cement board altogether).

Paint will probably interfere with tile adhesion. That is one of the primary reasons to use cement board. This is not a big deal because you'll probably still have enough grip, but why paint if it can only reduce adhesion?

Also, if it gets wet, mold can grow in there. Mold will not grow on cement board itself though, because there is nothing organic for the mold to eat. Also, if the paint gets wet for an extended period, it can get soft, and the tiles could literally fall off the wall. The thinset will stick to the tile, but there is a layer of mushy paint between that and the cement board.
 

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