Water spotting.

The term water spotting covers changes in the surface appearance of a coating resulting from the action of water standing on the paint film or substrate. The defect often takes the form of small rings, but also may be rough or discolored spots, or even streaks. Water spotting causes can be broken down into film surface and under-film (substrate) mechanisms. A quick look with a microscope usually enables the investigator to determine whether the water action was on top of the film or under it. Water spotting from underneath the coating often is a form of telegraphing. Residues from metal cleaning/pretreatment, power washing of plastic parts or other cleaning processes may telegraph through the coating. Another under-film mechanism can be interference with the cure or film formation of the subsequent paint film over spots of residues or contaminants. I once worked on a problem where snowmelt water leaked through a roof, somehow became contaminated with a basic material, then dripped onto freshly washed parts just before the drying oven. The subsequently applied acid-catalyzed topcoat blistered, boiled, and popped over the basic spots when it was baked.

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Film surface spotting may be caused by residues left when rainwater or carwash water droplets dry, boiling of water droplets on a wet electrodeposition coating as it is being baked, or droplets interacting with a coating that is water sensitive. Water sensitivity may be due to incomplete cure or retention of polar materials such as hydrophilic solvents and amines. The bake temperature may need to be increased, a catalyst may have to be added, or the coating may have to be reformulated to remove or reduce water sensitive materials.

Most spotting, whether on the film or under it, is due to water droplets. If you prevent droplets, then you prevent spotting. Where residue spots form, they are the same as the spots that can occur on glassware run through a dishwasher. Most dishwashing detergents contain surfactant rinse-aids to promote water sheeting instead of droplet formation. Such surfactants also can be used in cleaning solutions and rinses for metal and plastic parts and washing cars. When washing cars at home, the wet surface should be wiped with soft towels, especially in hard water areas. Unfortunately, there is not much we can do to reduce spotting from rainwater, which often is dirty due to air pollution and dust. Waxes and aftermarket coatings often make things worse by causing beading on horizontal surfaces, although they are not a problem elsewhere on vehicles. Fortunately, most automotive topcoats become more water spot resistant with time.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].