A lot depends on how bad it is. Generally it can be done by wire brushing with a drill, and lightly grinding the heavy stuff off the frame with grinder. I use POR15 to paint it. The company guarantees that it will stop rust if applied as suggested: 1. degrease any area that needs it, 2. spray with Metal ready 3. Paint with POR15. I have been using it for over 5 years on the recommendation of a friend that is a fleet mechanic. According to him, in all the years he has been using it he has never had to go back and do it over. On some trucks I have seen him do the entire frame and dump body. If exposed to sun light then you need to top coat it. The product is expensive but well worth it. One caution, wear protective clothing like a painting suit. If you get it on you and don't get it off before it starts to cure, you wear it until it wear off your skin, I know this from personal experience and there is a warning on the can :blush: .
There are other products also, the most popular alternative is Rust Bullet but I have no experience with it. Perhaps someone with some extended experience with it can comment.
Good luck,

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