Over the years I have seen many Defenders (in person) and many pictures of severe hinge rust. I have been able to avoid any hinge rust on my 95 D90 SW, even though it has lived for the past nine years less than a mile from the ocean in Mass (garaged, used mainly in the summer months). Here was the process I used since new: every time I wash it, I would spray the hinge joints with Boeshield T-9. I first got turned on to Boeshield when Griot's Garage used to carry it. For years now, I have only been able to get it at Marine/Boating supply stores (such as West Marine) and sometimes at the local Sears (in the tool dept.)
Boeshield was apparently developed by Boeing to spray in wing cavities. It is both a lubricant and corrosion inhibitor. The nice thing, when it dries it leaves a pretty durable, waxy film.
I have no affiliation, just really like the product. SInce it was worked my my Defender, thought I would pass on the info.