Causes

1. Surface contamination is the most common cause. Often the applicators surface preparation and cleaning is inadequate leaving traces of oils, silicones, water, dirt etc. Silicone contamination can be especially difficult to remove and requires extra vigilant cleaning and removal. Even in very small quantities silicone contamination from waxes and polishes, sanding dust, contaminated rags, household cleaning sprays and even cars being polished nearby can cause this effect.
2. Air supply contaminated with oil, silicone, dirt, water etc.
3. Hammer finish or other similar type coatings used in the same area.

Prevention

1. Always thoroughly degrease the surface before and after sanding and make every effort to remove all traces of silicone by thoroughly cleaning with the relevant HMG Preparatory cleaner.
2. Drain and clean the air pressure regulator daily to make sure that the intake is drawing on clean air. This also helps to remove trapped moisture.
3. Do not use hammer finish in the same areas as other paint finishes.
4. Any cleaning materials, car valeting materials, car maintenance materials should be sited as far away as possible from the painting area. These products often contain silicone and silicone oils and only tiny amounts can cause devastating effects.

Rectification

After the first, (affected coat) has been applied, applying further light coats slowly building up to the required finish sometimes can solve the issue. However in severe cases affected areas must be sanded down and re-finished.