An other alternative for a protective covering (i.e. 3M film) could be Plasti Dip. It's a spray on coating (like spray paint) and easily peels off. It won't protect from heavy object dents, but will keep the typical northern winter road grime off the finish. You can watch this youtube video for more info.
Fran, Mary & Zoey (silver Cocker)
2006 Itasca Sunrise 35A, 2005 W-22, Allison 1000 5 speed
'13 Ford Focus ST or '10 Prius on Master Tow Dolly 80THD-SB or '00 Jeep Cherokee 4-down