Do you re-prime after each successive sanding - 80, 180, 320?
After 80, yes.
After 180, yes.
After 320, NO.
320 is the grit that is going to prep the surface for your sealer or your paint. So you do not want to prime over the 320.
------- The way I do it is this: --------
I cut out all rust, weld in new metal. Then I block the entire car down with 80. I get rid of as much of the BS as I can in this fashion. I mark all the lows and highs and fix those. I do all my bodywork in 80. I block that whole car. I try to get the entire car as perfect as I can by looking closely and feeling with my hands.
Then when I think I have it perfect, I prep it with grease and wax remover, and shoot a coat of buildable primer. (The primer I use is recommended for use over bare metal, bondo, paint, other primer, whatever. It will etch itself to all that if prepared properly).
I give it a day or so, then I use my guide coat and start blocking it with 180.
The 180 step is really the last time I want to do any repairs of any sort. Fix it now. If you have to spot prime any areas because you missed something, or you hit bare metal, this is the time to do it. Make sure it's flawless when this step is done. Block it all 180. Make sure all the 80 grit scratches you created in the first step are smoothed over. You have got to get it done correctly now.
Once the 180 step is done and I am positive I have the car absolutely perfect, I move on and prep the car again with grease and wax remover. Then I shoot the last coat of primer. I usually use the same buildable primer but this time I thin it out according to the instructions. Thinning it makes it more of a sealer.
A day or 2 later, I start the 320 step. Carefully and slowly going over everything with the 320. This step is not body work. This step is PAINT PREP!!! This is the surface that you are going to paint over. The point of the 320 paper is NOT to remove any flaw other than a 180 grit scratch. The 320 sandpaper scratches gives the paint just enough that it can really grab onto the sealer/primer.
At this point, you put down that 320 paper and your car should be immaculately smooth and almost ready to paint.
-------- Paint -------
Painting is the easy part IF you let it be the easy part.
Get all your $hit ready in advance. Replace your compressor air filter. Drain your compressor (you should be doing that everyday anyway). I set my air pressure coming out of the compressor to 90 pounds. Attach your lines. Attach your water traps.
--- I usually run 100 foot of hose out of my compressor, I leave that hose coiled up, run it to my biggest water trap. This allows any water vapor coming out of the compressor to cool down and condensate into droplets which are then caught by the big water trap. Then I attach my 50 foot hose coming out of that trap. I run that hose to my gun. I have a small in line water trap at the base of my gun to catch any moisture that the big trap missed. Then I have a small regulator at the base of my gun and I regulate the pressure to right below 30 pounds ---
Make sure you have checked your gun settings. You will figure out how to set up your gun IF YOU PRACTICE! So PRATICE!
Always use a respirator. I always buy new filters for my respirator. Those thing do not last forever. You do not want to breathe the chemicals flashing off that paint or clear. You will eventually get cancer and you will die. Your family will sell your car and you will be eternally pissed. Don't do that. I need my Mopar brothers to be healthy.
Learn how to properly mix your paint. It's not that hard but it is easy to mess up.
Use the PROPER reducer depending on your temperature.
Make sure you buy enough paint beforehand... This sounds dumb but it's easy to screw up. Some colors don't cover as well as others. You may have to shoot up to 6 coats to get the coverage you need. Here's a tip... If you are shooting the exterior of a B Body, you are probably going to need about 2 gallons of pure paint to do the job. Figuring, mixing it with reducer (depending on instructions) if the paint you use says to reduce it 2 parts paint to 1 part reducer, you'll end up with 3 sprayable gallons of paint. Are you painting door jambs, inside doors, engine bay, inside trunk, bottom of hood, bottom of trunk lid, etc? You will likely need more paint than that even.
Clear coat - You are not going to want to skimp on this stuff. Usually sold in kits, the stuff I buy usually, 1 kit is usually enough to get my job done, but then again, you could use more if you want to do 3 coats in the jambs, under the hood, etc. 2 kits is more than enough but remember, you DO NOT have to use it all. It's OK. Each kit will make about a gallon and a half of clear coat when mixed properly. I like to get the "super" or "ultra" flow out stuff and I like to shoot when it's pretty warm outside. This lets the clear flow out nicely and minimizes orange peel which minimizes the whole colorsanding operation. Don't shoot more than 3 coats of pure clear. If you were adding a clear/pearl coat ok, but usually anymore than 3 coats of clear is going to screw it up and it's not going to come out crystal clear. READ THE INSTRUCTIONS. They will tell you what to do and what not to do. Don't take liberties. The people who make the chemicals know how it should be used.
I'm getting tired of writing this, lol. This is definitely not all you need to know but it should help out a bit. Give you some things to think about. It's just my take from the experience I have. I am not a pro. I am just a dude who messes around in my garage, trying to do the best I can and maybe help out a few guys in the process.