By Jim Kerr
Auto body shops across the country are changing the way they paint cars: they are starting to use water paints! Now these are not the streaky type of water paint you may have dabbled in when you were young. More correctly, the body shops are starting to use water-borne paints – paint solids mixed with water to make them fluid.
Automobile manufacturers have been switching over to water-borne paints for several years. The big push for this change has been environmental regulations designed to reduce VOC’s – volatile organic compounds – or as we simply call them, solvents. Solvents are mixed with the paints to make them fluid so they can be sprayed on the body panels, and painters refer to them as thinners or reducers, based on the chemical composition of the solvents. Now, body shops are becoming “greener” too by using water-borne paints.
Not all the paint materials used by a body shop are water-borne. Primers and fillers still use solvent-based carriers. The primer acts as a bonding agent between the bare metal and the colour coat. Fillers have more “body” to them, and fill slight low spots on the body panel. By sanding the panel after filler has been sprayed on it, the filler is removed from high spots but not the low spots, so the panel becomes smooth. Much of the difference between a quality paint finish and a so-so one is the time and quality of the workmanship spent getting those body panels smooth with filler. Even the best paints in the world look bad when sprayed on a wavy body panel.
Clear coats are sprayed on top of the paint colour coat to protect it and give it more depth. These clear coat finishes are also thinned with a solvent. The paint manufacturers have been working on new formulas to produce water-based clear coats, but few have matched the needed characteristics that solvent-based clear coats can provide. In the future, these too will probably be water-based.
So for now, it is the colour coat that is water-borne. Since a large percentage of the paint on an automobile is the colour coat, this has the greatest impact on reducing VOC’s. However, spraying a water-borne paint does present some new challenges. When the painter mixes the paint for spraying, the water must be pure. Filtered, distilled water is used so no chemical or mineral impurities are mixed with the paint. In actual practice, mixing the paint has become easier for the painter, as the water remains constant. With solvent-based paints, different solvents had to be used for different temperatures and humidity levels. Otherwise, the paint would dry too fast or not fast enough.
Spraying techniques are similar with either solvent or water-borne paints, although a different spray gun is used for the water-borne paints to provide the best finish. The biggest difference is in the environment the paint is sprayed in. Temperature and humidity must be controlled accurately when spraying water-borne paints or the water will not evaporate from the surface at the correct rate. Dryers can be used with water-borne paints, but instead of heat lamps, they use venturi nozzles that blow clean air onto the painted surface to help the water evaporate from the paint solids.
Once the water has evaporated from the paint solids, the water-borne paint is as durable as a solvent-borne paint. If you have any doubts, just look at the durability of the factory paint finishes, and almost all of these are water-borne. One of the advantages painters find with water-borne paints is that if a mistake or flaw occurs during the paint application, the painter can simply wash the panel off with clean water and spray it over again.
Another advantage, painters tell me, is that they match the factory paint finishes better. If you have had a fender or door painted, sometimes you can see a slight difference in the finish between panels when they are in bright sunlight. A painter explained it to me this way: when using solvent-based paints, the solvent is integral with the paint and as it evaporates, it leaves the paint solids on the finish sitting at many different angles. With water-borne paints, the paint solids float on the water and the evaporation leaves the paint solids all sitting flat. This provides a smoother finish and it requires less paint solids to cover the panel. The body shop finish now matches the original factory finish much closer. Time to dig out those water colours again!