the only kind of primer you should use is a 2k primer. Steer clear of the rattlecans and any kind of primer that has no activator. These kind never really dry or bond. I use dupont products, whether its the nason line or the chroma line, mostly because I have a charge account.
If you have rust, you need to remove it. The best way is to sand and grind. You should be able to tell by looking at it, if I needs cut out. If you have to cut it out, make sure to cut 1 - 3 inches past the infected area. Whenever you end up with a clean-cut hole, or a hole period for that matter, you should backpatch it with fiberglass matting or mesh. Be sure to prep the area to be patched with a heavy grit for a good bond. With a backpatch in place, it give you a surface for bodyputty to be used. Never use bodyputty to fill in holes. No matter how you prep it, it will always fall out in time. When using bodyputty, the surface should be prepped with no finer than 180 grit. The heavier the grit, the better the bond. I have never had much luck welding in patch-panels larger than a note card. I find it easier to use fiberglass, especially if I know the area is prone to rust. Fiberglass will not rust out.
Surface prep can be done any way you chose. All you need to remember is few tips. If our using a single stage paint (does not require a clearcoat) 220 grit is fine. If you choose to go with a basecoat/clearcoat then finish with 320 grit, unless the base color has metallic or pearl in it, then go to a 600 grit. I'm not sure what the guy above was talking about finish sanding with 2000 grit or finer, but I think he may of been confused with color sanding and buffing the present or newly finished paint.
When spraying your paint, use a gravity feed (material cup on top of gun) or a constant feed (pressue pot or pressure system) gun. Most guns spray best somewhere around 25 -40 psi. But be sure to do your homework on the gun you use. The paint most of the time will have psi guides too, but I tend to mainly go with the gun psi guide.
If you feel you need to color or wet sand after your paint has dried, be sure to finish with at least a 2500 grit. I usually go further and use Trizact 3000 wet sand paper. It looks like a thin sponge and you can put it on a 6 inch DA to cut down on time. Just be careful not to break through the clear or the color if a single stage. You can then buff this with a variable speed buffer and some buffing compound. Ther are many types of buffing pads and compounds to chose from.
When your project is completely done, stay away from the automatic and high pressure washers for at least three months. Give your new paint time to cure and wash it with a garden hose instead.
Hope some of this helps.