Deoxidene 624 is a simple and effective treatment, which comes at about £20 per litre. Best to swap a bottle of beer for a eggcup-full from your local friendly engineer.
A little goes a long way. Cee-Bee B-55 is a good metal brightener too.
I agree about the aerosol zinc chromate. Useless... you can wipe it off with Avgas!