Ok... here's the plan:
Go to a cheap hardware store and buy the least inexpensive and smallest diameter steel chimney brush you can buy along with 30' of steel 1/4" cable. Cut the cable in 15' lengths. Trim the chimney brush to the size of the box frame (believe me this will take a while!). Weld the cable to each end of the chimney brush. Now thread the cable through the boxed frame.
Now start drawing that brush through the frame, end to end. If it doesn't hang up, you've got it made. If it does hang up... you are on your own.
This will knock down most of the scale an debris. Then get some rust converter and pour it in, spray it in, however you want and follow up with some good paint (not Primer). I yearly spray used motor oil into my frame to cut down on rust and I've heard linseed oil also does a good job.
