The company I work for builds stainless commercial refrigerators for the restaurant industry and we do a ton of welding on stainless sheet metal.
We use a lot of air powered belt sanders, like this one from Harbor Freight:
http://www.harborfreight.com/air-belt-sander-97055.html
We use more expensive ones since they see 10+ hrs 5-6 days a week, but for the occasional use this one from Harbor Freight should work.