From an article I read:
Paint Blistering – Causes and Prevention
One of the complaints leveled at all paint manufacturers and suppliers concerns moisture blistering. It is probably one of the hardest problems to explain to the repairer on the body shop floor. This article will hopefully explain the problem in more detail.
With inclement weather conditions, there are bound to be some cars developing moisture blistering, which is more often than not blamed on the paints used. The body shop or painter rarely realises that the blistering is due either to; application, conditions in which you have sprayed the vehicle, preparation or indeed the weather itself.
The cause of blistering is saturation of the film by exceptional levels of atmospheric humidity, together with the presence of water soluble materials (sometimes primer), either absorbed into the film or present on the surface before painting. Blisters are formed when moisture eventually escapes from the film as humidity decreases and temperature increases. The pressures formed in the film are enormous and often sufficient to deform the paint film into blisters. Where very low temperatures are involved, moisture freezes in the film. This leads to weaknesses in adhesion which makes subsequent blistering much more likely. When water vapour penetrates the paint film it may set up a force sufficient to weaken the adhesion between coats of paint or even the adhesion between the whole paint process and the metal.
Many cases of micro-blistering occur when you repair or paint the vehicles under adverse conditions. You can minimise the chances of blistering by ensuring your paint area is kept clean and dry. During paint application, you need to use the correct thinner with each and every coat of paint. The use of one manufacturer’s product with another manufacturer’s thinners’ is not recommended, however good either may be. It is even worse to use a cheap quality thinner where the moisture content is often above specified levels. You should apply sufficient primer and topcoat in even coats. Ensure a sufficient primer film is left after sanding followed by a similar film weight of topcoat. Low film build of either or both weakens the paint structure and can very easily lead to blistering. Spot or localised repairs are very noticeable as the feather edge part of the area would undoubtedly have a low film weight. Low film weight is one of the most common causes of blistering, combined with adverse environmental conditions.
During the process, you make sure the surface to be painted is completely clean, free of grease and dry. Flash off and drying times as laid down by paint manufacturers, should be strictly adhered to. Too many jobs are rushed – the user may think he is saving time but in the long run, the vehicle may need some further rectification.
Over coating too early, especially in cold or humid conditions, does not allow the evaporation of thinner – this entrapped thinner eventually forces its way through the topcoat causing micro-blistering. This may not occur until some time after painting. If spraying, water traps should always be used in the airline system – compressed air must be dry. Drain and free the compressor of water regularly, once or twice a day may not be enough. Thorough flatting between coats enhances adhesion and reduces the subsequent risk of blistering. If wet flatting is used, constant changes of water are needed and all sanding residue must be removed by thorough cleaning. The moisture from sanding must be allowed to fully evaporate.
Ideally a workshop temperature of 60-65 F (17-18C) is important and the atmosphere must be clean and dry. Lower temperatures may increase humidity levels which are detrimental to the final result. The higher the humidity level the greater the risk of condensation which is a major cause of micro-blistering. For example the use of paraffin or calor gas heaters should be avoided as these create water vapour. The use of waxes on fresh paint work should be avoided before the film is fully dry, again the thinner would be trapped, thus leading to a soft film and possible subsequent blistering.
Basically then, poor weather conditions coupled with contamination and incorrectly prepared surfaces are the major causes of micro-blistering.