I also tried respraying two VERY RUSTY steel wheels!
On the first one, I spent around an hour sanding it down and then spraying it. However, the rust started to show within a few months.
On the second one, I spent around a week prepping it! Every night, I would put vinegar all over the wheel (absolutely soak the wheel in vinegar) and then in the morning, wash it out and sand for an hour, then do the same thing that night etc.
After around 7 days, all rust was removed off the wheel. Then instead of using the spray paint that I used on the first wheel, I used Hammerite rust treatment - this is a thick gloopy pink paint that you apply with a brush. After a few coats of this, the wheel was left to dry for a day, then I finally sprayed it in black! Two years later and not one bit of rust!
It is a very lengthy process 
The finish isn't the same smooth finish that I got on the first wheel (mainly because the pink paint is so thick), but the advantage is no rust.!
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