On Alfa number 15
AO Platinum Member
Join Date: Dec 2005
Location: United Kingdom
Underseal does seem variable, however on my last 156 the problem seemed to have been caused by a badly applied layer of bitumen sound deadening in the rear footwells.
Under the carpets at the back the floor is corrugated, and the layer of bitumen doesn't follow the corrugations very well, this allows moisture to sit between it and the metal.
The floor also has semi circular cutouts at the rear of the footwell, and these let water in in the first place, especially once the underseal starts to harden and crack with age.
Not the best design.
Having said that there are a few cars that seem to have escaped the rust bug, the earlier 156's seem to be better protected than the later ones.
The inner front wings and the front chassis spars (Inside the engine compartment near the cam cover)were also starting to rust on my W reg example.
You will never be cool driving a Vauxhall.