What condition is the underside in m8?????

Any surface rust will need rubbing right down and treating with some rust killer,then a coat of primer before you put your wax oil top coat on

Don't be surprised to find some areas that need welding under there though

Check around where the rear springs sit on the chassis and where the beam bolts up

If you want to do the job properly though you need to drop all the rear suspension off,strip all the old underseal off,prime it all,stone chip it,then paint it body colour