Scrape all loose material and debris from the damaged area, and then wipe down with water. Mix quick-setting cement and trowel it into the area. Work quickly—you only have a few minutes before concrete sets up.
Use the trowel or a putty knife to mold the concrete to follow the form of the object being repaired. Smooth the concrete as soon as it sets up. Buff with emery paper to smooth out any ridges after the repair dries.
Chip off the crumbled, loose, or deteriorated veneer from the wall using a cold chisel and maul. Chisel away damaged veneer until you have only good, solid surface remaining. Use care to avoid damaging the wall behind the veneer. Clean the repair area with a wire brush.
Clean up any metal lath in the repair area if it is in good condition. If not, cut it out with aviation snips. Add new lath where needed using masonry anchors to hold it to the wall.
Mix fortified sand-mix concrete (or specialty concrete blends for wall repair) and trowel it over the lath until it is even with the surrounding surfaces.
Re-create the surface texture to match the surrounding area. For our project, we used a soft-bristled brush to stipple the surface. To blend in the repair, add pigment to the sand mixture or paint the repair area after it dries.