I have used Electrox on the underneath of the car and floor inside the boot and interior, I brushed this on and then coated the underneath with stone chip;all the areas that I have painted have remained rust free for nearly 2 years whilst I crack on elsewhere. I am now considering brushing Electrox over the whole car and then going over again with an epoxy spray primer before applying a top coat.
Has anybody else done this? Perhaps this is overkill?
I did exactly what you have done. You can't really have overkill on a Mini!
I used the spay cans of Electrox on the inside and the canned Electrox with a roller on the underside. Then on the underside I rollered on Jotun epoxy primer (used on ships) then body coloured Raptor with a shultz gun. On the subframes I used Electrox, Jotun and spray can Frost chassis black.
The good thing about Jotun is that when used on ships they overpaint it on a regular basis, so its obviously not fussy about being applied to bare steel.