British (and European) Classics of the 60s and 70's.
Monocoque construction was new in the 60s & 70s and car designers had little experience of this new form of construction. Their main pre-occupation was making sure the car body was strong enough. Consequently, cars of this era are over-engineered and make good subjects for restoration. Little thought was given to how to protect the cars against corrosion and any thinking on this subject seems to have been mistaken. In particular, it seems that cars of this era had terrible water traps. The thinking at that time was to seal everything up as much as possible. Vents at the bottom of doors and in sills and box sections underneath are always inadequate, especially on Jaguars. The Mk2 for example has one little 1/4 inch drain hole for the whole of the" vestigial" chassis member which runs from the front bumper to the rear axle. It seems that no distinction was made between the harmful oxygen which feeds the rust process (which is going to be present anyway) and the beneficial currents of air which quickly dry out condensation and moisture. Apparently designers were terrified of water getting in from underneath the car and wanted to stop this happen by sealing everything up, when they should in fact have been making large holes everywhere to let moisture drain out and let plenty of air in to dry out the cavities!
Underseal Before the advent of the (generally excellent) modern polyurethane underseals in the late 70's, cars were undersealed with a bitumen type underseal. Bitumen underseal is pretty poor stuff and soon dries up and flakes off. Many cars of this era have been "tarted up" underneath with bitumen underseal. Bitumen applied to a rusty surface is an absolute disaster and will aggravate the problem while hiding it for two or three years until it starts to fall off in strips. When buying a car, if you are confronted with a car that has a recent application of bitumen underseal, even if it seems to be part of a legitimate restoration, I would walk away. By now any competent restorer should know that bitumen underseal is not the product to use. (By the way I am always happy to give free advice over the phone if you are thinking of buying a car.)
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