15ml will be plenty! I normally buy 30ml and usually you can get two cars done (C1), and thats the complete exteriors!

Dont do what I did will my last bottle of C1 though...knock it over by accident!!!! A VERY expensive mistake! Luckily there was enough to finish the Merc off!

With C1/C5 (they are the same thing), make sure there is no residue, polish, oils, previous sealents or waxes etc on the surface, otherwise the C1/C5 wont be as effective and will only last as long as the previous layer does (as its not bonding directly to paint)

Places I buy are usually from Gtechniq direct, Elite Car Care, or Cleanyourcar all very good and reliable companies to buy from.

If you buy from Elite, speak to Alex and let him know Amar with the CLS sent you and he normally sorts a little something out