I also own a Ford Ka with rust problems - in particular around the fuel filler cap and rear wiper.
Don't despair though - I found some "Rust Screening Rings" on Ebay specifically designed for these problem areas - bought both and they have really transformed my Ka and are really easy to fit.
Try the following link cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&;item=130432799232&ssPageName=STRK:MESELX:IT
You won't be dissapointed!
jamesflint
September 2010