Did OEMs wet sand and buff paint?

If you use a single stage urethane metallic you really can't buff it or you will mess up the metallic appearance. Solid colors in a single stage can take some sanding and buffing if you want to smooth it out a little. It all depends on how you lay the paint down. Using slower reducers allow the paint to flow out on the panel better but also introduce the chance of runs and sags. Longer flow out gives a better gloss and flatter paint surface with less orange peel. So, what you have is a trade off between how you are putting the paint down in terms of coat thickness, speed of moving he gun, and flow characteristics of the paint. Practicing your painting technique on some old fenders and hoods is a good idea, until you learn how to set up your gun and deal with the specific paint you are using.