Jay, it's not my thread but it seems like a natural extension, so here goes.

DP 90 is epoxy primer. Just as an aside, 90 is the black one you see most. DP is also available in other colors distiguished by a number change; e.g 40 is gray-green, 48 is white, 50 is gray, 60 is blue, 74 is red oxide, all now have the suffix LF for lead free (older versions of DP had lead). PPG refers to it as rust resistant, so it probably wouldn't hold out moisture by itself long term. Also it is not UV resistant, so between those two alone it's not suitable for a topcoat.................though lots of folks use it that way.

Usually the term 2K is in reference to two part (catalysed) primer/surfacer, or sandable, high build surfacing primer. As an example in PPG there is K36, a light gray acrylic urethane product. Epoxy primers like DP don't have much film build so aren't used the same as primer/surfacers.

Etch primers are also not primer surfacers, essentially do the same function as the DP, but have phosphoric acid mixed in to etch the metal, and convert very minor surface rust. Like DP it is a mechanical bond, although there is the chemical reaction from the acid.

Another note on the DP tech sheet warns against covering a laquer primer with DP and then later top coating with any of the DP compatible products. Experience has shown high chance of lifting the substrate.