Contaminants, including salts discussed below, can stick to your floor well enough to not be removed with a hose or brush, yet mysteriously 'pop off' after painting. For starters the salts work like dust and other contaminants getting between the coating and the surface. Without moisture, salts tend to form crystals, which can interfere with bonding. Experts claim these salts actually perform in a chemical sense more like a grease, damaging or destroying bonding. Unfortunately it gets worse.
Salts tend to attract water both from the concrete and through the coating.
Why Coatings Fail Simply not sticking to the surface is one of the more obvious coating failures and has been discussed above. Epoxies tend to yellow and chalk, other coatings, such as plastics, get brittle and crack. After too much UV the coatings will fail. Coatings are not as hard nor as solid as you might think. Some epoxies are more porous than other kinds of epoxies.