PabloCruise
SILVER Star
Napster,
My frame was sandblasted to start, but while I had it out from under the wagon, sprayed multiply products in it to stop and prevent the rust. Here's two pages of Eastwood's products. http://www.eastwood.com/paints/rust-solutions.html?p=1
I'd hang the frame up one way and spray in every hole, then hang the other way and spray. As a follow up every year spray a product like Fluid Film in the frame.
Maybe try power washing in there, just to get built up crap out, so the products get to metal. Going to be tough to get everywhere with the frame still bolted up. Once you feel you got it clean and dry, maybe some type of rust converter first.
Anything is better than nothing.
After the pressure wash maybe apply some phosphoric acid to the inside as a prep step?