What do y'all think about the spray on bed liner?
It seems to work nicely on skid plates and wheel wells, and it's really easy to apply if you prep the area.
Would something like that work for an undercarriage?

'02 Montero: OME suspension, ARB front bumper, homemade skid plates & spare tire Blitz can mount, extended diff. breather, K&N air filter, Optima Yellow Top, BajaRack, IPF lights, Goodyear Duratrac 265/75/16's, RUD 4x4 Grip chains