Old Fool said:
Why would you put acid in there?
Phosphoric acid does not stop rust, it only removes it, any acid left in place will activate when water gets in there and start eating things up, any oils included then the rust starts eating what the acid didnt.
There are numerous products he could use, epoxy being the best, but Ospho- not what I would use.
In my unending attempt to separate fact from fiction about Ospho and clear up the misconceptions about its use ,I just have to say something when someone is wrong...and the thinking that epoxy primer is some kind of cure all that it isn't...Epoxy may just be the best sealer against moisture and other contaminates but ONLY when used on clean metal
Spraying epoxy inside the frame over active rust will only cover it up ,trapping moisture and will actually speed things up.when the rust finally breaks through the epoxy ,it'll much worse that it would have been if you just left it alone,,,YOU CANT SPRAY ANYTHING OVER RUST and expect it to stop,it only covers it up while the rusting continues with a vengenance...
"NOTHING" STOPS RUST ,Its a natural process of nature reclaiming its elements ,rust (iron oxide) is a natural element steel is not,and nature always wins ,the best you can hope for is to slow it down to a snails pace.
while Ospho does remove rust pretty well it will also convert any microscopic traces of rust left behind which is good as long as its just traces your converting.The converting can also be a curse if you use it to convert heavier rust without removing it first..Its simply covering it up...
water will NOT reactivate the acid in Ospho and start eating the metal but will eventually wash of the protective coating and the rusting will start again ,naturally...Thats where the epoxy comes in its basically a sealer and seals out moisture,when used together AND properly they are an unbeatable combo
So on a boxed frame that you cant get at the rust Ospho isn't a good idea,spraying epoxy is also not a good idea if you cant remove the rust...
now you might be able to sand blast the inside of the frame if you could find some kind of a sandblasting wand ,Then ospho to convert whats left then epoxy ,the wands for fluids are available but I'm not sure about media blasting wands....There are some other ways to get the inside clean and coated but none that I know of are DIY....
Heres what I would do if you just want to cover the rust but don't want to make things worse later on ,use a black lacquer paint through a wand,it wont last long but it wont seal in the rust either, in a year or two when the rust pokes though, spray it again. these body wax undercoatings look like something worth looking into but I haven't actually used them...
Lets not confuse Ospho with Phosphoric acid,they are not the same,phosphoric acid is just an ingredient as it is in etching primer and nobody calls etching primer phosphoric acid.