Stucco Over Block
Q: "Can I apply stucco to painted cement block walls?"
The walls of my walkout basement are made of cement block, which has been painted on the outside. Can I stucco the wall to make them look better?
— Michael, Bird Creek, AK
Tom Silva replies: Cement block provides a good, solid base for stucco, but if you try to apply it directly to the walls, the paint will interfere with the bond. Stripping the paint would be a lot more work than it's worth, so you should first fasten galvanized metal lath (called diamond lath) to the walls with masonry fasteners. Then trowel on three layers of a traditional cement-based stucco to a total thickness of about one inch.
It's important to realize that stucco isn't waterproof. If your walls leak now, they'll eventually leak after being stuccoed. So before undertaking this project, be sure that you fill any cracks in the wall using hydraulic cement. And just as with any siding, you'll need to put flashing along the edges of the stucco to prevent water from getting underneath and causing it to deteriorate.