As far as really heavy building primers go the polyester based stuff such as slicksand are unbelivable. Sands like butter and is less prone to shrinking and can achive alot more build than a urethane primer. we use it to get all our cars lazer straight and then use a urethane primer to fill scratches and final prep, just remember as well as they work they must be treated in the same fasion as bodyfiller.
http://www.evercoat.com/productdetail.aspx?pid=28
This is a quick rundown of the order we do things at the restoration shop
-sandblast metal
-seal with 2 medeum wet coats epoxy and let cure 16 hrs at least
-sand with 180 followed by scotchbrite to remove all shine
-do filler work
-seal with 2 medeum wet coats epoxy and let cure 16 hrs at least
-sand with 180 followed by scotchbrite to remove all shine
-3 coats of slicksand let cure for one week(can be sanded in a few hours but letting it cure longer eliminates any risk of shrinkage, on 6 figure restorations there is no room for error)
-break and longboard with 180 till straight
-Seal with one coat of epoxy and let flash
-3 coats of 2k urethane highbuild
-block with 180-220-320-
-orbital 400-600
paint
-1 coat value shade sealer
-2 coats waterbourn basecoat
-3 coats premium appearence clear
-wetsand and polish
This is the process used on all of our museum and collector car restorations.