Now is the time for primer!
Before primer, put on clean gloves and thoroughly clean the surface to get rid of any grease. I use surface prep spray.
I use sandable primer, or filler primer. You can use what's known as a self-etching primer (supposed to be good for rusty parts), but I have heard conflicting opinions on that. The etching is done by an acid, so I am wondering about the long-term wisdom of leaving an acidic primer on your pieces. If you cleaned the parts properly, there should be minimal rust left anyway.
Filler primer is thicker than other primers. It will allow you to fill in small imperfections that are left behind by the sanding process. The key word here is small. It is not body filler. It will not make up for crappy surface prep, but it will help hide small sanding sins.
General rules for using spray cans:
1. Work in areas with plenty of light. Sun is best if you don't have a spray booth. Sunlight will allow you to see EXACTLY what the surface will look like. It exposes all sorts of imperfections you won't catch in crappy shop light.
2. MULTIPLE LIGHT COATS. You will be excited to start painting, and will want to start seeing results quickly. Spray paints are carried by a volatile solvent/propellant that evaporates from the surface of the piece, leaving behind the paint. If you glob on a crap ton at a time, the lower layers cannot dry effectively and will lead to a crappy final surface. Lay a light coat, then walk away. The bottle should tell you how long to wait between coats.
3. Nozzle distance. The bottle should not be too far or too close to the piece. Too far, and the solvent will dry mid-stream and will not adhere properly to the surface. Too close, and you will get runs. Runs happen when paint is applied too thick.
4. If you get a run, drip, or spot, let it dry thoroughly and sand it down. There is no saving it at this point.
5. Temperature and humidity. This is a big one. Painting in humid conditions (usually above 65% humidity or so) is the quickest way to get a terrible finish. Temperature should be below and above the range on the bottle. If it is colder where you are, you should put the bottle of paint in a can of warm water before you use it.
6. Shake the crap out of the bottle before you use it for around a minute.
7. Between applications, briefly invert the can and spray to clear the nozzle. This lets the propellant clear out the paint so it can't dry in the nozzle.