No need to start with 320 to strip it down, it will take way too long, use way too much sandpaper and you'll go through a ton of Beer in the process.
Start coarser and work your way down, it will go much quicker. On cars I start with 36 on an 8" rotary sander ( only strip 75% of the paint, avoid sparks from grinding into the metal ) , then 80 on the DA to remove remaining paint, then 150-180 DA and prime. If the truck is severely sun bleached you can probably start off with 80 grit and go up from there. See what works fastest without damaging the metal. Once stripped immediately apply either 1 coat of etching primer or epoxy primer before applying your 2K primer surface. NEVER use lacquer primer! Its garbage and will blow right off the truck if you apply it directly to smooth steel. If you don't plan to strip it down to metal I would start with 150 grit, then 220 then prime.