Why can't you install flooring directly over paint or other coatings on a subfloor?

The bond of the flooring to the subfloor is only as good as what you bond to. If you bond to old paint, sealers, polish, or other foreign matter, you are dependent upon the bond of that material to the subfloor to hold the flooring in place. Since we cannot determine how strong that bond may be, it is best to bond directly to the substrate.