Penetrol :P home hardware has it, not cheap, but you don't need much. It's linseed oil mixed with naptha and some other stuff to thin it - causes it to be very... capillaric... if that's a word. So long as any loose rust is removed, it will fill the sponge structure of existing surface rust and run through straight to the base metal if there's a route. Then the linseed oxidises becoming plastic basically - having filled the surface and any cracks / rust means it displaces any O2 or H2O from the structure and further rusting is impossible unless it becomes physically damaged. Boat builders have been using it on literally every surface (except sails), both above and below the waterline to waterproof everything - it's even been used to waterproof fibre board.
Abrade the rusted area and get rid of loose paint - then add a drop of the liquid, any remaining rust will almost instantly absorb it and you can watch it crawl through everything, you can then immediately add paint if you have it handy, no need to wait for it to dry. If you're doing large pieces and will be using something larger than a paint pen, you can pre-mix the paint and penetrol together before applying - works really well with primers to get them stuck on super well and right down to virgin metal.
View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...