Ref: G002, May 2004

One of the complaints levelled at all paint manufacturers concerns moisture blistering, which is probably one of the hardest problems to explain to the repairer on the bodyshop floor. 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 bodyshop rarely realises that the blistering is due either to application, conditions in which the vehicle is sprayed, 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, either absorbed into the film or present on the surface before painting. The blisters are formed when the 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 film into blisters. Where very low temperatures are involved, moisture freezes in the film and this leads to weakness in adhesion, which make the subsequent blistering much more likely.

One of the main causes of this phenomenon is osmotic pressure developed by the meeting under and in between the different layers microblistering. It may be that contamination is caused by moisture or even road salt. If a vehicle has been subjected to road salt and this is not very thoroughly flatted out of the film, osmotic blistering will occur when it is repainted.

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