Generally speaking, epoxy primer/sealer is something you would put down if the metal were perfect and needed no bodywork done to it. It gives a nice, uniform, smooth surface that is all the same color, which is important depending on what you are spraying over it-ie base/clear, single stage, etc. Some of the base colors are hard to get to look the same depending on if you are spraying one part that is white, one that is black, etc. That being said if you need to do bodywork, and it sounds like you do because you want to use 2k high build primer, I would metal prep the bare metal (always a good idea, I forgot to mention that earlier) spray some self-etching primer(good adhesion and corrosion resistance on bare metal), your 2k primer, sand and block your 3k until it is straight, a good epoxy sealer, and than paint. Make sure your 2k is fully cured before you seal it or it will bubble a couple months down the road as the solvents try to escape from the uncured 2k. As a side note if you need to use any filler, do that on the bare metal before the self-etching, assuming you are using something like z-grip, which I really like- great adhesion to bare metal and good corrosion resistance. Get that perfectly smooth, than self etch, 2k, block, seal, and paint. I'm no pro at this and I'm sure there are other ways to achieve the same result but this has always worked for me.
Hope this helps
Ben
