They need to be properly rustproofed with a decent wax after replacement
otherwise they'll rust out quicker than the originals.

I agree.  Also avoid

- Bad seals around doors (very expen$ive to replace)
- Stock steering box mounting bolt (tends to sheer off)
- Load-balancing suspension (nightmare to repair)
- Leaking main seals
- Sloppy shifter bushings
- Non-functioning electric windows
- Non-functioning fan clutch (easy fix; but annoying)
- Infamous "rocking driver's seat"

I owned a fairly well-kept low-mileage 6-series coupe in the Midwest. When I
bought the car, it had a light coat of surface rust all over the underside
of the front end, inside both front fender wells, and around the rear wheel
wells. The first thing I did was to pull the front fenders off, grind all of
the rust down to the bare metal, coat it in 3M Rocker Panel Coating, then
zinc-rich primer, then top coat, then urethane paint.   Within less than 1
year, after all of that of repair and rust prevention work, and with zero
winter driving, my beloved Coupe had begun to rust through the body in
several spots.  The body shop said it was probably rusting from the inside,
with water leaking in around the aging window seals, and dripping down
inside the body panels, where it would sit and corrode the metal.

I decided to sell it, because I couldn't stand to see a car rust.  The body
shop said there wasn't much I could do to stop it.  I had also spent
probably 100+ hours tracking down and repairing numerous annoying problems
endemic to the 633/635 coupe species.  I had removed the door panels 4x each
to remove and attempt to repair non-functioning window motors.  The car was
loaded with myriad small problems that added up to major annoyance.

Beautiful car.  Would never own one again.

WOE