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 body shop floor. With inclement weather conditions, some cars will develop moisture blistering, which is more often than not blamed on the paint used. The body shop rarely realizes 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 often formed when humidity decreases and temperature increases. Where very low temperatures are included, moisture freezes in the film (expanding its volume) and this leads to weakness in adhesion and may lead to blistering.

The physical force responsible for humidity blistering is osmotic pressure. Osmosis is described as the movement of water molecules through a semi-permeable membrane from an area of high water concentration to an area of low water concentration until equilibrium is reached. Because paint acts as a semi-permeable membrane, any dry salts on one side will draw moisture through from the wet side and the resulting expansion causes weakening of the paint layers or blisters. Trees manage to grow so tall because water is lost from the leave, osmosis causes the cell next to the leaf to draw moisture through into the leaf and so on. Water will be sucked 200 feet to the top of the tree.

Even more important if you manage to be imprisoned in a cell with iron bars for windows, if you squeeze matchsticks between the bars and the concrete and then add water, the matchsticks will swell due to osmosis and if repeated sufficiently, the concrete will eventually break. Thus any paint film will blister if soluble salts are present or if weather conditions are wet enough, regardless of when the vehicle was painted. Take a brand new production line car and place it in a swimming pool for two, months. Chances are that blistering of the paint finish will have occurred even though great care has been used to remove soluble salts (the use of demineralized water for its final rinse). The most common soluble salts are those present in water which fur up your kettle, calcium and magnesium carbonate. Sweat from the hand contains salt, sodium chloride. Road salts not fully removed and any of these materials allowed to dry on the panel surface may lead to blistering.

Vehicle repairers can minimize the chances of blistering by keeping the body shop clean and dry. During paint application, the correct thinner should be used with each coat. The use of one manufacturer's thinner with If another's product may cause incompatibility of soluble products, the use of cheap thinner which may have high levels of moisture from cheaper quality raw materials can lead to blistering.

Ensuring that sufficient film thickness of paint films is present after sanding, lower film thickness will allow faster moisture penetration. Low film thickness is one of the most common causes of blistering.

During the process, the surface to be painted must be completely clean and dry. If wet flatting is used, constant changes of water are required and all sanding residues must be removed by thorough cleaning. Moisture from sanding must be allowed to fully evaporate, about two hours at 1800 or more at lower temperatures. This is especially important with polyester products which should preferably be dry flatted. High or low air pressure may lead to uneven paint films resulting in porosity or air pockets and increasing the chance of blisters.

High humidity while spraying from cold workshops may lead to condensation forming on the fresh paint, latent heat of vaporization means thinners use heat from the panels to cause evaporation, this cools the panels and may show blooming of topcoats as moisture condenses on the cold surface, much the same way as windows do on cold evenings. This condensation is greatly increased with the use of paraffin heaters, where every gallon of paraffin burnt produces 1.25 gallons of moisture in the air.

Cold vehicles being painted or fresh paintwork being put out into cold damp air too soon is also liable to lead to problems.

Basically then, poor weather conditions coupled with contamination and incorrectly prepared surfaces are the major cause of micro blistering (small blisters). If impurities or moisture are retained within the paint films, pressure may develop resulting in blisters.

Cellulose and synthetic paint films are particularly susceptible to blistering, because their chemical structure allows easier moisture penetration. Two pack paints, because of the high degree of chemical cross-linking are less permeable to moisture.

When investigating a micro blistering complaint, it is important to gather the correct information before making any judgement. We know blistering is the result of osmotic pressure of water, but what has caused it? Firstly, identify where the blisters are occurring, at what point within the various paint layers. This may be possible with a scalpel, lifting off the top of the blister and viewing with a microscope. Often, flatting through the paint layers with fine flatting paper may reveal the blister edge. Polishing that edge can indicate depression craters in the underlying paints from the blister forces. Sellotape will often remove blisters or blistered paint from the failure point and examination may indicate the causes. Measuring the film thickness of the various layers and information about paint shop conditions, operators and processes, when the problem was noticed and any facts about the vehicle's history and use. The location of the blisters relative to the repair work can also indicate likely causes.

Having identified as many of these faults as possible, it is still often not possible to indicate the actual cause of the blistering. For example, I inspected 12 blistering Gas Board vans out of a batch of 50 painted by our customer. These 12 were not in sequence, but were awaiting change over in a compound. Blistering occurred between OE colour and repaint colour, within tolerance of paint spec.

The compound that the vans were kept in had tall conifer trees to the south and east and the weather office confirmed that it had rained each day for three weeks (South Wales is often like that) hence the vehicles had not had a chance to dry out. Those in service had been exposed to the same amount of rain, but had dried out while being driven, so did not blister.