I use epoxy over bare metal, over hi build primer, over body filler. I have never used it over old paint. I know that you CANNOT use epoxy over any kind of etch paint, such as etch primers. I really don't know whether you can use it over the original paint (is it lacquer?).
Yes, epoxy can be sanded. I sand it all the time. It sands easily. It's just that it's not a hi build primer, so it doesn't really have any "fill" qualities to it. It fills the swirls from an 80 grit DA sanding, but I think that is about the limit of it.
Etch primers have acid in them and you should not mix acid with epoxy. If you are going to use etch primer, then do not use epoxy for anything.
As far as SS being simpler than BCCC, the consensus seems to be that BCCC is simpler. I don't know because I've only shot a motorcycle tank in BCCC, not a whole car. I found SS paint easy to work with.
Epoxy is sprayed on with 2 "wet" coats ( or one wet coat mixed with reducer when using it to seal up al your work prior to painting). If you haven't handled a spray gun before, it is unlike spraying with an aerosol can- it lays paint down nice and wet (when the gun is adjusted properly). There won't be any "light" coats. If you desire a thinner coat of epoxy, then you add some reducer to it to thin it out. Once epoxy is on the metal, it's protected from moisture & corrosion. Other primers will not protect like epoxy- they are permeable and moisture will pass through most primers and allow it to go straight to the metal. This is why you see primered car & truck rusting underneath the primer- because primer isn't moisture proof. Epoxy is.
Hi build primer: basically sprayable body filler- it dries hard and thick and you can actually "shape" it with sanding blocks. It does not replace the body filler stage, but it takes your panels to the next level as far as smoothness and straightness. Just remember: the paint will reflect whatever is underneath it. If the panel is wavy, has pin holes in the primer, any small dents- it all shows up in the paint. Even sanding scratches come through, if you haven't done the sanding properly but moving up through eh different grits to remove sanding scratches from the previous grit.
The idea behind adding on more coat of epoxy over the truck before painting it is to create a barrier- moisture won't move through epoxy, but it will move into body filler and other primers. The paint is a barrier as well, so it's probably overkill in most instances to seal with epoxy, but that is what I see guys doing, just to be sure.
I can't add much more, because I only know what I have done. My understanding is that primer/surfacer is designed to be applied over old paint. Eastwood has a bunch of instructional videos on painting that might be worth watching.
Some light reading on primers:
Difference in Primers
