I've always used the wire brush-scrape and sand method.
I bought an electric DA thru Dewalt and it makes it so much easier.
No waiting on air and no noisy compressor running nonstop.
Do yourself a favor and be sure to use some seam sealer around all of the areas where the panels are joined together. Use a very small amount and pull it tight like installing caulking. I've seen a few that were wire brushed, primed and painted and then started to show faint rust lines around the edges of the panels. Sucks when that happens since there is no good easy fix...