You might look at Klasse sealent. I have had pretty good luck with it and you don't need but about an ounce per car. Make sure to read up on how to apply/take it off though. http://www.amazon.com/Klasse-High-Gloss-Sealant-Glaze/dp/B0002LBKG0
For my winter regiment, I use 1-2 coats of Klass with a Collinite 845 top coat.
I put like 30 coats of this once a year on all my cars. Even spray paint won’t stick. Don’t ask me how I know…
Klasse High Gloss Sealant Glaze 33 oz. https://www.amazon.com/dp/B0002LBKG0/ref=cm_sw_r_cp_api_i_52ADD332GJK5Z3NADHAE