DON'T use a spray can primer! they are not a high build primer and are really only good to give you a uniform color
and this does go for spary canprimes from the big paint companies too... if you look at there'* you will see that they're etch primers which means there only for spraying a LIGHT coat to help with addheastion of what ever will go ontop (all paints, and most primers are not direct-to-metal) the ONLY spray can primer i know that is a high build primer is made by BASF, It is a 2 part primer (primer & hardener) in which you have to break a seal in the can to mixe both parts to use it. but its only good for a day (it will harden in the can) To the best of my knowlage that primer CAN NOT be bought over the counter (profesional use only)
As for the primer you have to use a gun for, DON'T get the crap at the auto parts stores! because when you go to get it painted the painter will HATE YOU!! go to a carquest and get a RM (BASF), PPG, or DuPont primer only. they are the only ones the will not react badly to the solvents in the base coat of the paint process.
As for Application. make sure you feather/backsand your area to be primed with 320grit (no finer) and go atleast 4-6" PAST the outter edge of you repair with the first coat of primer, wait 5mins the allpy the second coat 2" IN from the edge os the last coat (smaller) wait another 5 mins the apply the last coat again 2" in fromn the last coat (smaller again) the last coat should be the same size as the reapir was in the first place
Doing that will insure you dont have any excesive build up around the outside to have to sand down and will hep makesure you get no sink rings later on after its painted.
Good luck, and happy priming