Top painter in scotland
Etch primer is only used on bare metal, prevents corrosion and adhesive properties
Hi build primer/primer filler is normally used to surfaces where it may be slightly uneven and requires a slight build to level the surface. This will then be required to sand and flatten back
Well normally according to most MDS primer needs to be sanded with 500 or upto 600 grit (dry)in order to have a scratch free finish. This depends on which paint you're using
If its rattle cans I would go as high as 800 grit (dry)
2k always have a better coverage than your normal water based and even solvent base paints. But to be sure I would finish up to 600 grit dry paper. It's better being safe than sorry
Etch primer is old hat now according to some, the high end resto places now use epoxy primer as it can be applied to the bare metal and filled over so there is nothing which is porous in contact with the metal at any point. When used wet-on-wet it can be a barrier coat and allow better adhesion also.
Also, if you are using 2k paint make sure you are aware of the health implications.