I'm cautious with all my cars which means regular inspections of the underside. With my Morgans, I know the undersides intimately, so I can soon spot any changes.
The primary risk is abrasion of the protective surface by road debris, which is more than you might imagine. Lots of modern cars have plastic mouldings under the floors, which help to protect the metal.
A good example on Morgans is my exhaust deflector plates. These are simple angled aluminium plates, which I fitted on each exhaust mounting, primarily to stop crud building up there.
The section under each mounting is horizontal. The leading edge is folded up at maybe 30 degrees. After only a few miles, the angled section has been grit blasted by road debris, whereas the horizontal section is hardly marked.
The other feature of Trads is that the body centre section stays clean. Only the wings and running boards take the crud.
It's worth checking the bottom couple of inches under the trailing edge of each rear wing. Here can get blasted down to bare metal very quickly.
