I've used OxySolv from Eastwood since the early '80s, and it's a great product. One big plus is that you can leave rusted parts in OxySolv for much longer than in an acid bath, without hydrogen embrittlement.
Another advantage is that OxySolv won't destroy your bare skin like strong acids will.
You can use it diluted with water to wipe on sheet metal to remove surface rust, and it leaves a protective coating to helps prevent re-rusting. Full strength, it's good at removing heavy rust if you give it enough time.