RUST, aka the dreaded tin-worm, rot, cancer:
The first thing to look for is obvious evidence of rust. All MGBs rust. The trick is to find it and stop it before it makes the car unsafe. MGBs were fabricated using unitized construction, or unibody. The car has no frame, per se, but is lent structural rigidity by the shape of its panels, all spot-welded together into a single load-bearing unit.
MGBs will rot, usually, beginning at the “dogleg” panels, moving inwards into the rocker panel. The lower rear section of the front fenders holds dirt and water and will rot from the inside out, as well as into the front of the rocker panel. Sometimes the exposed sections of the rockers themselves can show rot, but this is usually only in extreme cases.
Lets look at the dogleg panel [link]. This is the small section of the rear fender, in front of the rear wheels, that leads into the rocker panel, below the doors. Due to the original assembly and poor drainage in this area, this panel rots from the inside out. Any bubbling of the paint, or evidence that the panel has plastic filler or touch-up paint means you have a rotten dogleg. This is not disastrous. It is one of the things Bs do. It is a relatively simple repair involving removing the rotten section of panel, welding in a new one, a bit of bodywork, and re-spraying the area. Extensive rot in this panel is almost certain evidence that the rot has gone into the rocker panel, which runs behind the dogleg. This makes the repair much more involved. In the rare circumstances in which the rot is very localized, a patch can be welded into the rocker and then the dogleg repaired over it.
When the dogleg gets very bad, the rust will spread into the rocker panel itself, and even into the castle panel and inner rocker panels. These three panels form a longitudinal box section that gives the car its strength. When these panels rot, the rigidity of the car is affected. This is bad. The only repair is to completely cut the offending rocker sections out and weld in new ones. Keep in mind that there is nothing that cannot be repaired on a B. The question is whether it makes sense to do it.
The other areas prone to rot are the floors and trunk floor, especially on roadsters. Pull up the carpets and take a look; bubbling paint, thick peeling sections of sound deadening, funky, wet carpet padding or rust holes are signs that you need floors. This is another welding job, though recently I have started using some of the modern panel adhesives in certain instances. If the floors check out OK, also take a look at the four captive nuts into which the seats are bolted. Often these are torn out or are the subjects of weird repairs by previous owners. Try to rock the seat back and forth, if the panel is still sound, a bit of welding can fix these mounts and save the floor section.
Moving on, have a look at the lower rear section of the front fenders. This is another area that traps dirt and moisture, and rusts from the inside out. Since the fender panel can be un-bolted and removed, it is not as critical from a strength standpoint as are rockers and doglegs. Repairing this involves removal of the rotten section, welding on a patch and the attending paint repair. Bear in mind, the section of the rocker panel behind the lower fender can have the same problems as the fender and need to be addressed at the same time.