In wet weather, a small quantity of water vapor is absorbed into the paint system and will evaporate again in dry weather (osmosis). This process is normal and does not harm a perfectly matched paint system. However, poor processing of the undercoat materials will leave hygroscopic/water soluble substances (salts) behind as contaminants which cause a local concentration of humidity lifting the paint film into blisters. Blisters can occur in many sizes, patterns and frequency and can form between individual layers or beneath the entire paint system.