I didnt take the whole underside to bare metal. The shell on mine was pretty good and the only bits it needed were the rear chassis legs (exactly the same place as yours) and the rear corner sections of the sills (again the same as yours). I wire wheeled all the stone chip from the areas that had surface rust but left the rest original. I just thought there was no point disturbing it.

I used red oxide on all the bits a welded and wire wheeled and then painted the whole underside together. Here was mine.







This was it just cleaned off.



After red oxide and grey paint.