Mine on my 68 are undercoated from way back.
The whole truck was Zie-Barted when new or close to new.
The stuff under the wheel wells has turned brown.Looks like crap!
I've had mine out for a/c work and re-painting and no problems.
So,if you drive it,the dirt will stick to it unless whatever you use gets hard and dry.
Rhino-Liner might be the best under there.
Mike.
