You've got a DA, so that's a good start. Get some 45, 80, 150, 320 sanding discs. Start with the 45 and get the layers of paint and primer down until metal starts peeking through. Use the 80 to get it down to the metal. Now you can assess the situation. Any rust holes, where are the low spots, where are the high spots?
Do your metal work. Patch any holes (you'll need a MIG welder of you don't have one), pound up the low spots from the backside, pound down the high spots from the front side.
Now clean the metal, then clean it again, then spray the metal with epoxy primer (and only epoxy primer). EP is a waterproof sealer and the base for the filler work you will do next.
Get some 3M Platinum filler (part number is 11020 or something close). Mix it and lay it on just like drywall mud or tile grout. Just a medium layer will do, 99% of it will end up on the floor anyway. Sand with 120 (with a sanding block!!) crosshatch pattern, until it's level. Spray with 2K primer and evaluate your work. Most likely, there will be a couple more spots to touch up. You can use glaze putty or more filler. Respray those areas with 2K primer. Sand the entire area with 120, just light pressure.
Here is where opinions will vary. At this point I will shoot it with a 4K high build primer. Featherfill G2 is my go to. This is thick stuff that requires a 2.0 nozzle to spray out properly. Some say 1.8, I disagree. This primer will fill swirls, small scratches, and small pinholes. Sand this down with the 320.
Now you are ready to spray the paint. Personally, I always put down a layer of sealer before paint. Up to you. If you use sealer, sand with 400 before paint.
On the areas that already lookok good, you can sand down with the 320 until the clear coat starts to disappear. Then go back over with 400 and spray the new paint on it.
Are you going with single stage or base/clear? There's more discussion to be had, based on that answer.
