Overlays work best on a high-quality substrate. Decorative Concrete Institute in Temple, GA

Again, since you often will not have much influence over the substrate beneath an overlay, you can only hope that you're starting with a floor that will properly support the overlay. The biggest thing to watch out for from the substrate is moisture transmission.

In the third book in his series, "Guide to Concrete Overlays and Toppings," Bob Harris discusses how to test for moisture-vapor transmission (MVT). What we come back to—I feel like a preacher, here—is the importance of a low-water content mix that doesn't shrink and has low permeability that will therefore prevent much moisture movement. "If you have an impermeable substrate," says Harris, "you'll have much more success with the overlay."