all bodywork should be done before you prime anything.
If you sand down to bare metal you want to use a sealer/primer before you use regular primer.
Start out using 220 grit all over the cars body, fix any damages, and spot prime those areas to be sure they are flat and correct.
Once you have the whole car primed, you can wetsand it using 600 - or - 800 grit wet/dry sandpaper, use with a bucket of warm soapy water, make sure you keep it wet, and listen for scratching sounds, if you hear them stop and rince the body.
Before you paint, use a tack rag to remove any dust sirt and dibris from the body.
Paint the door/trunk/hood jams first, as well as wheel openings.
Then the outside, starting from the roof down.
As far as the paint, your best choice is going to be a basecoat clearcoat system, either from PPG or dupont.
Use a medium reducer for this. It gives a little longer flash time, but goes on a bit easier for amatures.
After your paint is complete the following day, you can remove the paper, and cut the clearcoat with a 1500 or finer wet sandpaper, again using a bucket or hose to keep the body clean while sanding.
Once the body has been sanded out all the way and there are no more shiny spots, you want to use a cutting wheel on a buffer wheel, using a rubbing compound of medium grit, this will begin to bring back the shine.
After 30 minutes and a complete wash job.
Now replace with a polishing bonnet, and use a polish of your choice to bring the real gloss to the top, and you will have a show room shine.
Remember to look at the compound and polish to be sure there are no canuba waxes inside. You will want to stay away from carnuba wax for the forst 6 months !