To ensure as little paint as possible is wasted when the paint is being transferred from the spray gun to the vehicle bodywork a humidity level of around 70%rH is needed. This will help avoid the problem of paint evaporation and in turn reduce paint costs.
Vehicle engines are tested in a number of different humidities and temperatures to ensure the engine functions correctly in different conditions.
During sanding it is important to maintain a relative humidity of 55% moisture to reduce the electrical static charge of the surrounding air so that the dust created is of a similar electrical charge to that of the car body surface. This causes the dust to flow off the bodywork rather than be attracted to it.