How to remove pits from stainless steel vessels

We are full time outside contractors at a adult nutritionals, infant formula facility. Once a year stainless vessels are inspected(large 25' dia. 40' tall) and pits and stains marked then we have to enter vessels and remove pits and stains. Stains are not a problem to remove pits we use small pneumatic end grinders with compacted sand cloth wheels on them it works but is very slow in doing such a large area. Do you know of any hand held machine that may speed up this process?

Thank You,

My concern is for the vessel's integrity, especially if pitting occurs in the same region year after year. Perhaps an HAZ (heat affected zone) from fabrication welding is vulnerable to corrosion. Pits could be cleaned and filled by either brush electroplating or welding, or the entire pitted zone could be coated by thermally sprayed SS, but the decision belongs to the client's engineer-of-record. ASME pressure vessel codes and FDA regulations are likely involved.