Painting in humidity

BMW April 2022

Weather plays an integral role in our lives in various ways and for perfect finishes, rising temperatures and humidity are important considerations. With summer rains, higher temperatures and increased humidity, your paint provider and paint shop manager should adjust your system to counteract changes in the environment. Even if the temperature in which you are painting is mild-to-moderate, excessively low or high humidity can affect the overall quality of your task. Changing the speed of the reducers and hardeners, and the use and balance of accelerators and other additives should also be reviewed.

Bubbling and blistering occur when there is no longer adhesion between the paint and the surface and moisture got trapped underneath the coating. This also happens when temperatures rise above 32°C and paint dries too quickly.

The following conditions may be common indicators of a temperature problem in your summer system. 

Loss of gloss, blush 

Lack of humidity control causes areas presenting a white haze or low gloss. This happens when the substrate temperature is below the dew point and humidity condenses on the wet paint due to the cooling effect of solvent evaporation. Check for unsuitable reducers, poor circulation in the drying or film thickness being too high or low as causes of the loss of gloss. The problem can be solved with proper humidity control.

Solvent pop

Solvent popping, or pinholes, appears when the solvent evaporates too fast from a wet film (paint) and the increasingly viscous liquid paint does not flow into the resulting void during oven cure. Proper oven staging will enable the slow release of the solvent. To overcome this problem, consider the following:

  • Use an acid salt to slow the cure and enable solvent release in an acid catalysed system;
  • Increase flash time before bake;
  • Use slower evaporating solvent;
  • Apply additional thinner coats to build film rather than fewer thick coats for spray applications;
  • Use a dehydration bake lower than the boiling point of water followed by a second higher bake to cure for waterborne coatings; and
  • Use lower Tg resins with lower dry film thickness.

Orange peel

Orange peel may develop on painted and cast surfaces which resembles the surface of the skin of an orange It is occurs when paint is applied at high viscosity or under conditions harmful to proper flow and levelling. You should check the technical data sheet and adjust the paint to a proper viscosity. Atomise or reduce the size of the paint particles that are sprayed. Atomisation can be achieved by reducing the amount of paint being sprayed at a given time, increasing the pressure used on an airless sprayer, or using different equipment that allows atomisation more effectively.