So I found over the weekend that I have surface rust inside #21 and #22

anyone have any idea how to deal with it without cutting it open?

Thanks

 

You really can't. If it's not bad you can spray something like Rust Check/Krown/Fluid Film in there or some other oily type undercoating and it will drastically slow down the progression of rust. If you really truely want to stop it you'd have to drill out the spotweld, separate the pieces, sandblast everything, paint the inside with epoxy primer and then weld it back together, and again fill with some sort of oil based undercoating

 

Spray por51 forget about it maybe reinforce with with rebar if you want