the EPA is pushing for less voc's in the base coat, water base is the only alternative right now. Some states it's mandatory, other states it isnt yet.
Here in Alberta waterborne has been required for at least a year. However, there is another alternative to the waterborne base coat. There are several manufacturers now with eco-paints that are a low VOC solvent-based base coat. They're getting to have lots of colors in them as well, and they apparently spray exactly like the regular solvent-based base coat did. Something else to look out for too, if you're absolutely set against waterborne.
