Whether or not you need primer depends on what kind of paint is on it and what you are going to use. If you are going to use a base coat /clear coat( I recomend you do) you will haveto put a good coat of a Urathane primer over the current paint or it will lift.( unless it already has a base/clear on it) It will even lift Laquer primer...the urethane primer has a hardener with it...good stuff and it fills in nicely because it is thick. I wouldn't use the DA on it if it were me unless it has a ton of paint on it. Wet sand it with a good straight paint stick. It does not matter if it is flat or curved...a straight paint stick will follow the shape of the car. I would use 220 or 320 to block the paint down, with the stick..block sand it dull and if you see metal on any high spots STOP. To really make it look good it should be block sanded at least twice..maybe more depending on how straight the car is.I would put a good heavy coat of the Urethane over the sanded paint and block sand it until you see any high spots then primer it again..then sand it back down again...as many times as you need to get it straight..THIS IS the best way to get it straight...but don't end up with too much primer on it..use some common sense as far as that goes. When you are happy with it being straight wet sand the primer slick(but not too slick) with 400 wet/dry. Once it slicks up you can hose it down with H2O and it will look like paint...you can see how straight it is that way. Once that is smoothe use a silver scotch brite pad to scuff it up a little then wipe it down with a rag with some wax/ grease remover (I use laquer thinner but don't soak it) blow out every nook and crany cause dust and or water will come out in the paint .Tape it up then tac it off good.
The base coat will dry very quickly and be dull like satin paint..wait a half hour..tac it off then clear coat .
Base/.clear is THE ONLY way to go in my opinion. Any blunder made when spraying the color can be wet sanded 10 minutes later and resprayed..... great stuff. Another benefit you can color sand it the very next day...so if your paint job looks dry as long as you have enough clear on it you can block sand the clear with a stick with 1500 or 2000 grit paper the buff it back out...I always do this no matter how slick it is because it always makes it look better..gives it that mirrored deep look....