The best product I.ve used recently, and it does what it says on the tin (or plastic bottle) is Jennychem Hyperwax. It was about £20 odd cant remember. Put a little in a foam gun spray it, leave for a minute an towel dry. Its beads water for 2-3 months.
Never had anything like this. The 2 lutre bottle is gonna last a long time. Do the windows s well, dont have to buy RainX ever again.