Not always a guarantee but worth a try, I make up a rapid rust solution for spot fixing though colour may be a problem. It is a simple mixture of Salt, White Vinegar, Hydrogen peroxide. Let it rust, card, then pour boiling water from a kettle over the area. It is one of those processes in the realms of "Suck it and see". Keep an eye on things if the atmosphere is at all damp rust will form before your eyes so do not leave it on too long things are very dependent on the strength of the Hydrogen Peroxide you use.
