Well usualy their is no metal left. The right way to fix bad rust is to cut it away. If you grind it, it does come back...or you run out of metal to grind.
I just sent Brian the part number for the metal piece below the rear windsheild on the W126. They are cheap like $75, and when I bought mine Mercedes only had like half a dozen left in stock, period. So if you plan on having W126's for awhile it wouldn't be a bad idea to get one or two just in case.
I figure if I never use it, I'll sell it for $200 when Mercedes doesn't have them anymore.
