2k can mean a lot of different primer types. 2k (2 component- dang Europe in there use of K to spell words -lol). Meaning the product is activated and cures by chemical reaction. But generally when someone says 2k primer, they are most often talking about a urethane primer. Epoxy, urethane, and polyester primers are all 2k
Urethane filler primers are porous. They generally have more fill and sand easier then epoxys. Some have higher build then others- you find different terms like primer-sealer, primer-surfacer, high build. They don't shrink a lot like a lacquer primer would, but do shrink to some extent, so its best to allow to sit as long as possible or get some uv exposure to allow shrinkage to occur before sanding-block sanding. Not really the best choice over bare metal, although there are some direct to metal types and epoxy-acrylic hybrids.
Epoxys are little slower curing, but are tough when they do, and non porous. Best Choice over bare metal for corrosion protection and adhesion. Epoxys don't generally sand as easily or have as much fill. I don't know for sure if its the slower cure or the resin that makes it stick so well. They can usually be wetsanded pretty well the next day or two though. I think epoxys shrinkage is minimal. The most unique epoxy I've used has been Southern polyurethanes by far. More fill then most, can even be dry block sanded often the next day, and drys glossier then most which helps to see how straight bodywork is looking. Some people only use Epoxy primer, which is possible depending on how good the bodywork is and well finished off.
Epoxy can be used as a sealer properly reduced, and make a good sealer for greatest long term holdout and durability. Many urethane primers can also be reduced and used as a sealer as well, although not as good a choice is epoxy based in my opinion. The product data sheets for whatever you use should say if recommended as a sealer, and proper way to mix and use.
If the entire car is 2k primed, been all worked out, sanded to perfection with the proper final grit, no reason really to spray a sealer before paint. The time it would be advantage to use a sealer, as epoxy sealer over urethane primer for better long term hold out and durability, if your painting over a sensitive substrate, or you have different colored areas to get everything one even color to paint over-or your sealer is the appropriate shade that by spraying it coverage will be achieved with less coats with your paint or basecoat color.
Both epoxy and urethane primers require that baremetal and surface be properly prepped by sanding, blasting, ect. The only type that may not is if it includes some sort of acid etch component, and etch primers are old tech, 1k, and could be a recipe for disaster-softened and staying gooey underneath from solvents in products applied over, acid component causing problems with other products or coming back to bite you in the future. Too many risks for rewards IMO. Start by properly prep of all metal-sanding, experienced media blaster using proper material, ect, physically get rid of all rust, and start off with an epoxy primer IMO is the best way for a long lasting job with least chances of problems or incompatibilities occurring now or in the future.