Ever have a product that works TOO WELL?  Rain-X is one of those.  As an
  anti-wetting agent it is simply superb, so good it doesn't leave enough 
  water on the windshield to lubricate the wipers.  Now rubbing a rubber
  wiper across a dry windshield will cause skipping and abrasion and wear 
  of that wiper pretty quickly and sure enough, that's what happens.

  A wiper floats on a thin film of water and slides easily when this film
  is present.  Without the film, the rubber drags on the "dry" surface and
  the spring action of the wiper arm and associated mechanically
  articulated magic gizmos cause the wiper to skip.  Each time the wiper
  blade skips it leaves a microscopic pile of rubber on the glass.  Each
  pile begets another pile and like topsy just grows until you have this
  dark patch on the glass.  Easily removable with a number of nasty
  solvents, this residue is a clear or not so clear indication of fast
  wiper degradation soooo just run the wipers on intermittant or use old
  decrepit wipers that work poorly on non treated glass.

  As a prank, surreptitiously rain-x only the driver's side of someone's
  conveyance, passengers will approach panic as they can't see at all while
  the driver has excellent visibility.   Stuff also keeps the snow buildup
  down and makes it easy to get the ice and road groad off.
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         ___  / __   __/   ____/         Al Knoll HP Performance Technology /
        /    /      /     /             Center    Roseville, CA, USA 95747 /
       _____/      /     /             916.785.5317    (Telnet 785-5317)  /
      /           /     /             email:  alf@hpptc44.rose.hp.com    /
   __/         __/   ______/         ___________________________________/