Depends if its been undersealed before. Mine had tonnes of underseale on it when i got it so i found the best way was to soften it up with a heat gun then a scrape it off. After you've done any welding that needs doing then a soft pad on the angle grinder will get rid of any surface rust followed by a wire wheel on the drill for hard to reach places. I then used red oxide primer or etch primer will do followed by stonechip, then primer, and then paint or hamerite.

Check my resto as ive not long finished the underside