This a common problem. Stone chips damage the protective lacquer, water gets in under the surface of it and the aluminium oxidises.
Basically you need to carefully rub away ALL of the old lacquer and re-lacquer the surface. This is a diy job. Use the search facility and you should find lots of info. For my penneth the worth though, mask everything off well and try to use a "scotchbrite" type pad if possible working along the pattern.
Avoid wire wool as although very good any bits that happen to break off and get trapped will show up as rust at a later date.

Colin