An even sneakier and worse rust hazard lurks under the front fenders. All along the joint between the outer fender and the inner fender (that separates the wheel well form the engine compartment) is a foam seal blocking a gap of about 1/2 inch or so. This seal is supposed to seal off the air pocket that extends from the base of the headlight hump to the hood shut line. (Hard to explain, but go look at the car and you'll see what I'm talking about). This air pocket extends pretty much all the way from the A-pillar to near the grille. Over the years this foam seal may break down, especially if it isn't reinforced with an outer layer of undercoating. When it goes bad it allows moisture into this cavity and your first clue will be a sudden sprouting of rust all along the 'valley' between the headlight humps and the hood edge, and the bottom edge can rust out as well into the engine compartment.

Don't ask me how I know :(

Rusty '71 1800E - Either future MIG welder exercise or drivetrain donor (to a 445???)