Body prep is the most important part of painting. If the body work is not correctly done, it will show in the paint. Even the best painter in the world can not hide bad bodywork.
I have been painting for over 30 yrs and if there is one thing I highly recommend using is epoxy primer . It covers and allows you to do bodywork over it there by sealing bare metal much better. Also once a body is done in epoxy primer it is sealed against rust.
I painted my wagon in filler primer and then topcoated with epoxy primer this past Sept. The filler primer was $400 for the gallon and the quart of catalyzer. The epoxy primer was $400 for a gallon and quart of catalyzer as well. Now you also add in bondo, sandpaper etc ,it can be expensive. But it is still cheaper then having a good bodyshop do it.