Just bought the Meguiars convertible & cabriolet weatherproofer for my car last night. I actually witnessed a bird crap hit it 5 minutes after spraying the full can on the hood so I flung water on it & it just fell off, no stain or infact water. Expensive but worth every penny. Seems the hydrophobic formula repels almost anything, not sure how long it lasts though. Just thought I'd share my find for anyone interested.
http://www.meguiars.co.uk/product/224/Convertible-Cabriolet-Weatherproofer/
http://www.meguiars.co.uk/product/224/Convertible-Cabriolet-Weatherproofer/