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.
A single layer of any coating runs the risk of including tiny pinhole areas of non-coatings. These 'leaks' permit chemicals to attack the unprotected surface as well as the bonding interface. Each coat is a contrasting color to aid in spotting thin or missing spots. Older epoxy formulations were generally more brittle than many of their modern counterparts. Different coatings handle abrasion and wear better than others.