Chemical degradation (water as solvent and chemical reagent).

Corrosion (water as electrolyte, an ionic current carrier, supporting electrochemical corrosion reactions).

Blistering (consider osmosis under the influence of soluble salt surface contamination. Note that a paint coating film acting as semi-permeable membrane is permeable to water but impermeable to species dissolved in the water).



Osmosis principle - schematic (click on image to enlarge)

Freeze-thaw damage (volumetric expansion of water on freezing).

Mechanical wear by erosion, impingement and cavitation (mechanical surface impact).

Hygroscopic stresses (arising from absorption / desorption of water by paint films) and their effects on paint adhesion to the substrate.

Supporting microbiologically influenced degradation of paint coatings.