The Spray Gun
The last significant change in the design of the automotive paint spray gun was the switch from siphon-feed regular-pressure guns to the high-volume low-pressure (HVLP) gravity-feed design. Some gun manufacturers still make siphon-feed guns with HVLP output, but the majority of painters are using gravity-feed guns with the cup positioned on top.
The use of waterborne paints requires a slightly different version of the HVLP gun. All of the major spray gun manufacturers now offer a line of spray guns designed for use with waterborne paint. Gun components must be resistant to corrosion due to the constant exposure to water. Sata offers their top-of-the-line Satajet 3000 HVLP Digital with corrosion-resistant components and a handle plated with chrome. Needle sets are stamped with a "WSB", which indicates use for waterborne materials and will feature a special 1.3mm fluid tip. Other gun manufacturers like Devilbiss also offer waterborne spray equipment; their popular Gti HVLP gun has been outfitted with special internal parts, a distinctive blue handle, and is re-designated Gti-W, for waterborne.
Since most painters have already invested a good chunk of change in spray guns for solvent paints, you might be wondering if it is absolutely necessary to update your guns for waterborne use. Well, we experimented a little and sprayed some waterborne primer through a non-waterborne gun and found that although it will work initially, we noticed an immediate deterioration on the surface of some of the internal components as well as the gun handle itself. And this occurred even after a thorough cleaning; all moisture was removed, gun-cleaning solvent was sprayed through the gun and it was oiled right after use. It wouldn't take long for the corrosion to affect the precision function of the needle and other mechanisms inside the gun. So we highly recommend investing in a spray gun designed for shooting waterborne paints.
When paint materials are properly reduced and ready for application, they must be poured from the mixing vessel into the gun cup. This is a process that can transfer unwanted particles into the gun, which can and often does have a negative affect on the atomization and fan pattern of the material being sprayed. Worse yet, unfiltered particles have a way of passing through the gun and splattering on a most conspicuous spot on a car. To reduce the chances of this happening, filtering, or straining, solvent-based paints, primers, and sealers into the gun has been a standard procedure for many years. Most paint suppliers usually furnished complimentary paper filters with the purchase of any paint. Those cone-shaped disposable paper filters are still good for your solvent materials, however, you might need to set them aside when loading water-based paints into your gun. Since many of those paper strainers are constructed with a water-soluble adhesive holding them together, they may literally fall apart when used to strain waterborne paints. Also, the filtering mesh is most likely insufficient for screening out foreign particles. While solvent-based materials are typically filtered at 200 microns, a waterborne paint strainer must provide filtration down to 125 microns. 125-micron paper filters are intended for use with waterborne materials such as Envirobase HP and will most likely not come apart, however, you should check with your supplier prior to use.
The drying characteristic of waterborne paint is entirely different from that of solvent basecoats. Heat via climate control or heat lamps is no longer an effective means for accelerated drying (unless of course, you are spraying solvent-based coatings). Waterborne paints require air movement to effectively evaporate the water content in the paint film. Spray booths can be retrofitted with large air dryers for high volume work and complete paintjobs, or painters can use handheld air dryers for smaller spot repair jobs. Hand dryers such as the Sata Dry Jet features a fine screen to prevent foreign matter in the airstream from passing onto the surface, and consume approximately the same amount of air as a spray gun, and the pressure can be adjusted to suit the application.