I simple way of explaining a 2k primer or sealer is that it is one that cures chemically by adding a hardener, activator, catalyst. Whatever they happen to call it. It does noy rely on air to cure like a primer that doesn't have an activator that comes with it, A good example being lacquer primers. Thats why you also can get lacquer primer in spray bombs, but a 2k you cannot, it would harden up in the can in a matter of hours if the 2 parts were mixed. The 2 part primers have a pot life, and if you leave them in a spray gun without cleaning it the gun would be pretty much trash if it sat too long. But in the old days when lacquer primer was mainly used at a few shops I worked at they would leave the lacquer primer in the gun overnight and use it the next day. And if the lacquer primer did dry up a little shooting some lacquer thinner through the gun would usually take care of it.
I third the advice, shoot epoxy primer on baremetal and then shoot 2k urethane primer for fill. You only need 1 or 2 coats of epoxy primer.