I'm no expert, but have some experience. The catylized primers still shrink significantly over time, so less is more. These days I DA off the old paint, straighten the metal as best I can, spray a K200 type high build primer on it, and block it down as thin as I can get it without cutting through to the high spots. It's hard to know where you are sometimes without a paint thickness meter, so it is a great tool to have if you are doing several cars.
Once its flat, I seal and top coat or Base/Clear. There are also occassions that require some body filler.
If you color sand the entire surface, it will off-gas faster than with the shiney surface. Polish and enjoy.