Most cars built in the Seventies didn't have much in the way of factory-applied rust proofing, and in this respect the Marina was typical. Not only was the Marinas metal badly protected but poor weather seals allowed the interior and boot to fill with water. Bizarrely, as production progressed, the standard of construction seemed to get worse, with paint and steel quality deteriorating throughout the period of production. The worst rust spots are the sills - the rot starts at both ends then spreads into the floorpan and wheel arches. Inner sills are tougher than the outer ones, but once the wings start to rust they can disappear quickly. The following pictures are from 2008 restorations published with the permission of the owners, not all cars are this bad, but it is worth seeing what can be hiding under a layer of grime and paint.
Scroll through the photos to the left to see some examples.
The rear edge of the front wings (A post) is a notorious rot spot, along with the area above the headlamps, the headlamp panels themselves and the front and rear valances.
By the time you get there, the corners of the boot may have disappeared, with the jacking points and rear spring hangers likely to have gone the same way. Various panels are available from the Club.
Some Marina variants (SDLs and TCs) were fitted with stainless steel wheel arch trims, which harbour rust. Whether the trims are fitted or not it's likely that the arches will be looking frilly; especially the rear ones. The inner rear arches also tend to rot badly, with the section above the bump stop reaction plate (in the centre) usually being the first to go.
Doors rot readily, especially the bottoms, but as all the body styles share the same bodywork in front of the B-posts, the front doors of all are interchangeable - with a bit of tweaking, an Ital door can also be made to fit. Estate rear doors have a different window frame from the saloon, but finding second-hand replacements isn't hard. Door hinges wear badly, especially front ones. Saloon and estate lower rear hinges are the same as the front ones, so good second-hand ones are available. Regular lubrication will help to prevent wear.
Leaking windscreen seals lead to damaged floorpans. If the car has a vinyl roof, examine it for tell-tale signs of rust underneath. Vulnerable areas are those at the edges of the covering, where water has penetrated to start corrosion in the metal below.