Do not do that! Most dremel bits other than the wire brush are liable to punch a hole through the frame, and the wire brushes are much too small to mess with. Perhaps if you are really careful and good with dremels you could get away with it, but without any offense to to you cantor, the very fact you had to ask leads me to believe you shouldn't use a dremel. What I would do is get a fine grit diamond file and use that. Then as eubi said, prime and paint.

As far as the chain and stuff, I'd pull them off to protect the frame, then use strong cleaning chemicals from the hardware store. One word of advise, however much time the the bottle for the cleaning agents say to wait before you spray the parts off with water, follow it exactly. Acids can disentegrate metal fairly easily, I wrecked 1 or 2 window AC units in my HVAC days using acid compounds to try and restore them.