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Gonna put down sheet vinyl in a m.h. master bath.

Plan is to install 1/4" plywood over the Cres-Tek mushboard.

Apparently marine ply isn't available in 1/4". The flooring installer said they use plain old AC grade plywood, even in bathrooms. I'd like to do more as far as waterproofing. Any recommendations for painting something onto the plywood before the vinyl goes down?

I'll sand the Cres-Tek high spots, but wondering if I should trowel on some leveler for the low spots or not. I imagine it depends on how obvious the low spots are, eh?

Personally I don't like doing a second layer over mushboard. I would remove it and go with a single layer of 3/4" plywood. The reason is that if the mushboard does get wet & disintegrate you will have a void under the top layer.

Greg

Pull the mush board and install 3/4 plywood, use grade aproved for vinyl install.
If your fasteners don not hold in mush board they will start popping and come through vinyl

Definitely agree with both Greg and yakima4$ above. Smooth sanded 3/4" plywood screwed and glued down, fill screw holes with floor leveler then lay the vinyl and you will never have a problem again. Get rid of that mush board.

Ron