Coating 304 and lesser grades of stainless isnt going to work as one single pinhole in the coating .... and you wont have any coating.
What you are going have to do is mirror polish the stainless (to about 5-10 micro inches), then get the parts electropolished. A mirror finish will retard the rusting. Any roughness, old weld scars, joints is where this 'stainless' is rusting - am I correct? If so your only alternative is to mirror-polish.
Mirror polishing is typically done with successively smaller and smaller grits of sand paper (down to 1000 grit) to remove ALL scratches and other surface roughness .... then power-buff on a high speed fabric wheel using 'tripoli' a mixture of clay and diatomaceous earth as the 'polishing rouge'. Usually just the mirror polishing is enough to prevent rust, but then you have to occasionally 'wax' the exposed surface to keep it from getting 'wetted'.