Painting Water Resistant Wallboard

Dear NH,
I have just done a renovation on my bathroom walls and installed a waterproof cement board on the walls and ceiling. I'm tiling the walls, but I don't want tiles falling on our heads. Can I paint with regular latex paint over the cement board?

Dear L,
Painting cement board is not really a standard practice, but it can be done. Seal the seams with standard wallboard tape and compound. Since the product does not have the tapered edges of wallboard, you must feather the compound out at least a foot on either side of the joints to give the illusion of a flat surface. Use a water-based primer/sealer for the first paint coat, followed by a coat or two of a mildew resistant bathroom paint. A washable eggshell or semi-gloss latex paint with a mildewcide added would be my distant second paint choice.

If you find the finish on the cement board to not be to your liking, you can always use a texture paint to mask any defects. Of course, at least one coat of a mildew resistant paint should be applied over the texture paint. Texture paint is too thick to mix a mildewcide into, and it will definitely mildew over time!

Water resistant wallboard would have been a better choice for a painted surface. It is designed to be more easily cut and finished like regular wallboard, but withstands much higher moisture levels without damage. There has been some controversy about this product, and most tile contractors no longer use it within shower and tub enclosures as a backer, preferring the various tile backer board products.

As an aside, being a curious-cat, I decided to do a simple product test of water-resistant wallboard a few years ago. I took a 6" x 6" piece, placed it into a large pan of boiling water and let it boil for an hour. The result... there was no deterioration in the paper whatsoever (though it did absorb water), and the solid core showed not the slightest bit of softening.

So the question that arose in my mind was... why is this product frowned upon? Though not a complete answer, I realized the sad truth was that contractor error or irresponsibility could be part of the problem, not the wallboard. I have done at least 50 tile repairs in fairly new homes... less than 12 years old... caused by water leaking behind tiles in enclosures. What I have found consistently is that the contractors used regular wallboard, not water-resistant wallboard, in these failed jobs! I have personally never seen water-resistant wallboard properly installed on walls or ceilings fail due to dampness in residential conditions.


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