Pressure washer.

Let dry completely.

Undercoat with spray wand. Remove all the access plugs and stick the spray wand inside the frame and soak it good.

Don't go mudding if you like your frame.

 

Thats probably the best advice yet

 

if you really wanna keep your frame brand new from the inside why dont you clean the inside as best you can let it dry with a heat gun then spray that great stuff inside the frame cause its water proof and wont let water in and keep- it nice and clean dangit now i wanna do this

 

I just used a garden hose. Took all the plugs out and sprayed in all the holes to flush out all the mud and crap. Let it dry then I sprayed in chainsaw bar oil with an engine cleaner wand.

 

I would get a pressure washer or borrow one for the job. Really isnt much better for getting in there. My buddy really wanted to rust proof his inner frame because he lives in salty new york and daily drives his rig in the winter without getting daily carwashes. He took rustoleum paint and a thick twine rope, soaking the rope in the paint first then tying it to a string already through his frame. He dragged it back and fourth adding paint and sloshing it around. It was a messy, non-perfect, time consuming affair, but I gotta admit, the results were pretty impressive considering he didnt remove anything to get it done. May or may not be worth all the effort in the long run, but you can be damn sure that you should paint your whole frame if your body ever comes off!

 

DAY OF THE DEAD!!! THE ZOMBIES RISE FROM THE GRAVE!!!

why bringin back an old thread??

 

I would disagree. Sealing it up won't stop or prevent rust it will actually make it worse.
The reasons frames rust is crap/dirt gets trapped inside and condensation and salt accelerates the rust.

Keep the drain holes clear and wash your frame out once in a while.
Also the motor oil trick works too but it's a pita and messy.
Best bet you could have someone spray chain oil lube underneath your truck and inside your frame after you clean it and let it dry with a wand and a sprayer. It may not be environmentally friendly with the motor oil trick either.
You want something that is gonna stay up there a while and thick motor oil only last a short time.

They used to have those rusty jones treatments back in the late 80's and early 90's that used a kind of parrifin wax to seal everything underneath your car. It prevented quite a bit of rust probably why my truck isn't as rusty as most on the road there is still some of that stuff under the hood.