This rain away stuff:

http://www.merproducts.com/acatalog/Rain_Away.html

(which I got from a Wilkinson store for much less than that online price) is great - no need to use the wipers again ever (well, until the effect wears off, but lasted a month so far).

My ombrello is only "just" starting to lose its effectiveness now, after being on my car for approximately 15000+ miles... Luckily I have a spare pot, so I reckon I'll be using the glass cutter soon, and putting a fresh layer on.