Not sure what type PPG paint he was using, but I suspect it was an acrylic enamel and not the polyurethane like PPG Concept that uses a polyisocyanate hardener. I used PPG single stage Concept on my 230Sl, and I can tell you that it was hard enough to color sand and buff within 24 hours. The 22 line Glasurit is similar to the Concept paint using the same process to chemically cross link and harden the paint so that it can be color sanded and buffed the next day. With a two stage process i.e. Base coat/clear coat, typically an acylic base coat is used to color the car. This coat does not contain a hardener and is very similar in application to a lacquer paint in that it dries quickly to a matte finish. The gloss is obtained by overspraying this base coat with a clear coat that contains a hardener so that it can be color sanded and buffed. Done properly a base/ clear job can look almost like the single stage job.