Just to deal with your comment about 2K and epoxy, epoxy primers are ground coats applied directly to bare metal and/or over filler work. They are often also used as sealers over pre-existing finishes that will be painted over. For example the PPG product is DP XX (the XX means there are a variety of different numbers that denote color, e.g. 90 is black). These primers are meant to seal the metal/filled surface in preparation for sandable primer/surfacers which the term 2K generally refers to. Again to use the best known product, PPG, that would be the K XX product (again the XX represents a number code that denotes color, e.g. 36 is grey). These are generally polyester based products that give a "high build" for the purpose of block sanding to smooth the body surface in prep for top coats