Clean back to bare metal with angle grinder wire brush, coat the whole exposed area with Kurust or similar, then etch prime, then prime/filler prime, then flat back to satisfactory finish before finally applying top coat and lacquer. This should last well.
Don't leave exposed metal naked. As soon as you've brushed, get the Kurust on (not too thick), wait for it to dry, then get it primed and covered as soon as possible.
The combination of the Kurust and etch primer will give it the best grab and possibility of resisting further rust.
Take your time and ensure everything is coated and theres no way for water to seep into your repair area later on. I've repaired loads of surface rust in this way, with no problems.
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