Lostmarbles, would it be a better idea to put the mudflaps right behind the tires to keep th e mud from sliging up into rear inside frame railing which is where these are prone to rusting? Mine had them right behind the wheels in which I am going to replace them with better flaps.
I think it is best to leave ready-access so you can get in there with a hose Rob. I think the way you propose would inhibit access and require a "less-clean" mounting method for your mudflaps. (IE. - more complicated)
Having the right "grade" of rubber is important too. It needs to be heavy + floppy. (Light or stiff material is hopeless.) Look at what the big rigs use!
