por15 do one called metal ready, kills the rust then leaves a zinc coating. not sure on durability of it tho. but even vinegar will work on rust if you soak it long enough, get some tissues soaked in it and drape over problem area, put cling film over the top. 
Vinegar seems to be working!
Only had white wine vinegar in the cupboard, it removed a bit.
Bought some malt vinegar today for 13p, so if it works **BARGAIN**
Also ordered a new fuel cap.