[SilverMk2]

If there is nothing wrong with the paint I wouldn't recommend stripping it. If so just scuff it and feather out all the chips and scratches with 320, then prime and paint. If the paint is checking, flaking, or just really bad you'll want to strip it. They make special paint strippers for bumpers or you could have it walnut blasted. If it is this bad you might want to locate a bumper refurbishing place. They have some around here that boils the cover and fixes any damage. When you get the bumper back its pre-primed and looks perfect. They are actually easier to repaint than the factory replacement bumpers nowadays.

 

[SilverMk2]

What kind of primer you use is really dependent on what paint you use, i.e. Dupont will have a recommend Dupont sealer and this Dupont primer for this Dupont paint. You definetly want to use flex add in the paint to prevent flaking. This is probably what happened to the bumper you have. If I were you I'd find a auto paint store locally and I'd go in and talk to them about what you are going to do. They should pretty much be able to tell you what you need to do and buy. Most of these guys are pretty knowledgeable about what they sell.