First off, 220 isn't "glass smooth". Not even close! If you are going to use those high dollar paints, you might want to invest in some more sand paper. Also, if that guy says it won't shrink over time, I would let him paint it, as I would bet against him. I did not see where you said if you are painting in a paint booth or not, but most of the products now days are designed to be used in a paint booth, if you are doing collision work, where it is expected to be done fast.