The metalizers need to be sealed with the Testors sealer first.

Matt I scuff everything between coats. DBU is some of the older PPG paints (before the DBC stuff we use now) I have also used the DCC paints these are catalysed and could be used without a clearcoat, or with the clearcoat mixed in. I don't know how they would work as flats though as they have a pretty high gloss on their own.