If you were to attend class at the SoCal PPG Training Center, instructors Paul Stoll and Frank Ramos will tell you that volatile organic compounds are a class of materials that include most of the evaporative solvents used in automotive refinishing products. We won't get into specific ingredients since most of them are known by their chemical brand name, but suffice to say that VOCs are found in all primers, sealers, paints and toners, clearcoats, hardeners or catalysts, reducers, and cleaning solvents. There are non-VOC solvents available; however, they tend to evaporate quickly thereby increasing the cost factor and do not always provide the best results.
A volatile organic compound is measured by its weight. For example: A typical gallon of automotive refinishing reducer contains 7 pounds of VOCs. Considering that a gallon of water weighs in at approximately 8.3 pounds by comparison, 7 pounds of VOCs per fluid gallon is a lot! Basecoats that use VOC-containing solvents have a VOC content of 6.0 to 6.6 pounds of VOCs. The new rule for most areas in California (and with many other states soon to follow) is 3.5 pounds of VOCs for basecoats and 2.1 to 2.8 pounds for primers, sealers, and clearcoats.
Water vs. Solvent
OK, that heading might be somewhat misleading since this is not a competition per se. There are numerous and significant changes between solvent basecoats and waterborne basecoats in preparation requirements, spraying techniques, and overall visual characteristics of the coatings. These are the areas that painters will be most interested in. Preparation includes all of the steps required in order to get the surface ready for the basecoat and clearcoat. Cleaning, body repair, metal prepping, material removal, and abrasive treatments, primer, and sealer application all fall into the prep category. There are many variables involved in each stage of preparation, and not all of them may apply to the topic of waterborne application. So what we will focus on here are the methods that might require some relearning on behalf of the individual painter.
One of the main differences that you will notice with waterborne basecoats is that its dry film is thinner than that of solvent paints by as much as 50 percent. The higher solids of a solvent basecoat tend to fill more of the surface irregularities such as sanding grit marks and even small pinholes. Because the thinner dry basecoat film will conform to the surface and reveal those irregularities, final sanding with finer grit is an absolute necessity. It is not uncommon to machine-prep (always with a pneumatic dual-action random orbital sander; no fixed orbitals or electric sanders!) a surface for solvent basecoats with sanding grits in the 220 to 240 range, with hand sanding done with 320 to 400 grit. In preparing a surface for a coat of waterborne Envirobase HP, PPG recommends hand-sanding most new OEM replacement panels with 400 grit (by machine or hand) prior to sealing for waterborne basecoats. If no sealer coat is required, you can simply scuff-sand the panel with 600-grit prior to base coating. As with all standard prepping methods, you should avoid breaking through the factory sealer coat and exposing any part of the bare panel.
A non-sanding sealer is applied onto the sanded panel for two primary reasons: to provide a uniform substrate for the waterborne basecoat color coverage and to enhance the quality of adhesion. Instructor Ramos emphasized the importance of maintaining the correct sealer coat thickness as well as color uniformity in order to achieve a good color match. Too thin of a sealer will most likely cause the waterborne topcoat to lift (lifting appears as a wrinkling of the paint coat and is caused when solvents are either incompatible or are allowed to penetrate a sealer of insufficient thickness). Once a proper sealer coat has been applied and allowed to flash dry for the recommended amount of time, the waterborne basecoat can then be sprayed.