I remember spending ages getting my A-frame cleanish with a wire brush and drill and hammeriting it. I was persuaded by a friend to take it along to a tame fabricator who put it in his blast cabinet. I was amazed what a single pass of shotblast would do, there was no trace of rust or my Hammerite, just a clean matt finish.
But if you cant get them to come to you then you are a bit stuffed. I guess you just remove the all the components you can from the shell and get them done elsewhere , mechanically clean the flat bits of the shell, the little gaps and holes can be done with rust removal liquid/gel/foam stuff.
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