You don't really have to get ALL of the rust off; you just need to get the loose rust off so that the surface will be smooth and not cause the paint to flake.
Go and buy a wire brush attachment for a drill and use that; be sure to wear safety glasses and long sleeves, because there will be tiny bits of hot metal flying around. Once it is smoothed down, give it a treatment with a rust retardant primer, then two coats of rust retardant paint. You could even finish it off with something like Rustoleum LeakSeal, which is actually a rubber coating.
Be sure to treat the inside of the pole as well, if it was open or has holes drilled in it; even if it is still rusty inside this will help prevent it rusting outward from under the paint. Also, make sure that if it was open that you cap it; a pvc endcap and some caulk should do it.