No matter how clean the hands may seem, there is always some dirt, grease, oil or perspiration which will cause blistering and poor adhesion. For example sweat from our hands will be made up from (predominantly sodium chloride [NaCl], but also potassium, urea, creatine, creatinine, lactate, and phosphate) this is why in motor body assembly plants, cars are never directly touched with human hands. After being chemically
cleaned - the operators wear cotton gloves. It is a good practice for all painters to follow.
In wet weather, a small quantity of water vapour 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 and patterns with varying frequency and can form between individual layers or beneath the entire paint system. In dry weather, most blisters will recede.