A lot of people use fish oils etc to rust-proof their Defenders but the fish oil of course won't stop rust that has already started.

On different sites one of the Dinitrol products have been recommended to convert and seal rust within bulkheads, doors etc. Unfortunately the stuff is hard to get in oz.

There is a very similar product available here called fe-123 Rust Converter which I have just used on my defender. I used a similar product years ago on my old BJ40 Toyota and it was brilliant. It converts rust and forms a plastic coat suitable for painting and sealing the surface. I did parts of the bullbar on the tojo that were lightly rusted and without any painting 5 years later there was no sign of rust.

The fe-123 is quite pasty and I diluted it with water before spraying it into door cavities on my deefer. Sprayed the back bumper bar that had virtually no paint on it and was a rusty orange colour. Half an hour later it was black and clearly sealed and ready for painting.

Looks like a good product. You can find it on fleabay or :

fe-123 Rust Converter

Best Wishes

Will