Truck topper is about 15 years old and has the normal top-hinged window above the truck tailgate.  The piece of weatherstripping that seals the window-tailgate joint is still in useable condition.

  The problem is that with age, the weatherstrip leaves very annoying and substantial black residue on anything it touches.  It is removable and I've tried cleaning it with window washing (ammonia based) products and ArmorAll.  It helps for a month or so.

  Any suggestions for keeping it clean?  TIA.  Stetts