Hope it is in good condition then, remove the rear trim panels, this gives access to the inside of the rear wheel arches, inspect well in there, and if all is well; treat.
My preferred method is to use an industrial paint, this is sprayed on and many thin coats are put on as it can be directly applied to bare steel, but does require air fed visor to apply.
When this is dry, i spray an industrial version of Waxoyl into this panel, this is not Waxoyl, but a compound we put into steel for use offshore on oil rigs and the like.
Similar principles can be applied using other materials.

Over the rear end, particularly above the fuel tank, i clean off thoroughly, as this area traps dirt and acts like a sponge, i shotblast then use a Waxoyl underseal. This self heals, and takes several applications to get my desired result, Dinitrol also do a suitable range of products.

Do inside the chassis and sills, insert a hosepipe inside and thoroughly wash out, and treat; this way it will last for years, a lot of work but the payback comes in the future.