[Argess]

Any undercoating that makes a skin is bad on metal as if mositure gets in, it can't escape. I'd use spray on black undercoating under the fibreglass wheel wells (even fibered roofing compound) to absorb rock impacts, and use a lanolyn based undercoating for the floors and frame, stuff like RustCheck, which seeps into the cracks and crevices.

Actually, since I only drive in summer, and on nice days, I didn't undercoat. Besides, if you have an FE, you'll likely have "automatic undercoating".....I do !.....LOL.