Depends on a lot of variables there.
Why is it necessary to take it down to bare metal? Is the exciting finish peeling off, really old, been paint too many time before, rust spots, or just needs bodywork done all over?
Its best to try & keep the factory primer/sealers on the metal if ya can.
In most cases the most appropriate method is to use a rotary grinder with a soft backing pad w/40 grit to remove most of the finish almost down to the metal, then go over that with a dual-action sander w/ 80grt.
Be careful not to stay in one spot to keep from heating up the panels & warping them.
If it only has a few coatings, skip the grinder & just go with a da sander w/80grt.