I used Mar-Hyde Undercoating (product # 4111) which is not a rubberized coating. It was put on over a year ago and it still doing it's job. I had one outrigger at the back of the fenderwell which had started to rust out. I sprayed it, plugged it with some type of body tar and sprayed over it with Mar-Hyde again. Looks fine.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system