Salvage project - rusty trampoline frame- input needed

Hi folks,

I need some input on a salvage/repair project. I have recently been given a 12' second hand garden trampoline for my boys to enjoy. It has been previously outdoors with no protection for 15 years.

It seems to be salvageable, but I'm a little unsure what to do with the legs. They are made of a single long piece of steel tubing which has been bent into a "W" shape, with the two bottoms of the "W" in contact with the damp ground.

They have heavy rust where the pipe bends and made contact with the ground, and the way it is designed lets water into the top of the legs which has sat inside the pipe, causing internal rust as well. This seems to be the critical area to protect, because it will rust through sooner rather than later. Here is what I have come up with:

1) Hit the worst of the external rust with sandpaper/wire wheel etc... as per the other parts.
2) Pour rust converter (phosphoric acid) into the pipe, with the end plugged.
3) Swirl the phosphoric around occasionally to ensure full contact/coverage.
4) Drain and catch the phosphoric acid to reuse.
5) Wash the inside of the pipe with masses of water to neutralise/dilute the residue.
6) Dry the inside of the pipe thoroughly - hot air gun, blown in from one end.
7) Tip some kind of paint/sealant into the pipe, swirl it around until all internal surfaces are fully coated, then allow to drain.

Ultimately I'm hoping to get another 15 years life from this.

I see one or two of these a year at our town's transfer station metal recycling heap... I've got a set of those legs, too bad there's not a doorway from my Boston to your Boston that I could just pass them through.
I've seen the "W" type, but also eight short straight legs that connect to a second ring on the ground? I feel like these designs allow for temporary placements on soft ground such as a lawn, and dragging aside to mow below. I imagine that more permanent installations with altered legs could be made.

I pass a house on my commute, that has a trampoline "installed" in the yard. There seems to be bricks encircling it, and I can assume that it is over a shallow pit to allow for the trampoline to do its thing. I feel like this could be an option if you can't salvage the legs?
I also think it could be safer? since an errant jump/bounce wouldn't be an additional 30 inch fall to the ground. Trampolines here seem to be equipped with a net to keep this from happening.

Your plan seems decent though. I've seen some of the "W" legs with half the tube rotted away (in the heap), yours doesn't sound that bad?

Correct, my tubes are not as bad as you describe. They are mostly sound metal, but I don’t think they will stay that way if it is ignored.  Sadly digging a hole is a non-starter here, but it is a good idea where it is possible.

I envision a chicken tractor:  flip it upside down.  remove springs and remaining fabric.  surround with any form of chicken "wire" (I keep a few rolls of the plastic stuff for temporary/seasonal things like this). add a cover of tarp material.  it should skid on the round frame. Or it could be a nice garden frame and cover for spring.  That tubular material is pretty stout.  

I had someone in another thread recommend to me a trampoline frame as a trellis for kiwi.

It is going to be used as a trampoline for my boys to bounce on.