I am about to overlay slate (12" x 12" x 1/2") onto an exterior porch and a new step and slab. The porch is 55 years old, covered by the home's roof, and is in amazingly great shape. My only concern is that the surface of the older concrete is rather smooth. (I put a broom finish on the new concrete.) I will powerwash the old concrete, but I'm wondering if I should also grind the porch surface before laying the slate. I have a man-size buffer/grinder or I could rent something a little more industrial. Perhaps the answer is simply in the thinset to use. Any pros out there with insight? Thanks, Jim
Grinding would likely make the surface as smooth as it is now. I would give it a couple of acid-washes using Muriatic acid. The acid will slightly etch the surface to provide more grab for the thinset.
Use the proper thinset also. I would suggest using Mapei's Kerabond mixed with their additive Keralastic.
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