Hmm... I am not sure that the hidden parts don't need polishing for corrosion resistance. The info. about SS needing O2 to maintain its corrosion resistance is quite correct; but as the hidden surfaces will most probably in an O2 deficent area but will still be damp (at least occasionally), then it will need all the protection it can get and a highly polished SS surface provides better corrosion resistance that a rough surface. Of course if the hidden surfaces are completely dry then maybe it is OK to leave them rough - but I have not found a completely dry area on my boat
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As for hand polishing, you seem on the right track, I normally go to 400 or 800 grit on the linishing belt before going to the cutting compound on the sisial buff. After that, a polishing compound on a stitched cotton buff and finished with fine polishing compound on a loose cotton buff. It is only then that you get a high polished finish.
But it is a lot easier to take it to a shop and get it electro-polished and that (I am told) is far superior for corrosion protection - and looks better too.