My bumper became loose on my "84 300D. I was suprised it was bonded from the factory with no bolts holding the bumper to the struts. Apparently the bumper had oxidized behind and around the rubber strut fixture in contact with the aluminum bumper surface. The oxidation was a white powdery substance and that is what seems to indicate the bumper is aluminum. There was no "orange rust"
Scraped away the loose oxidation and used aluminum Naval Jelly to remove any residual oxidation around where the rubber strut would re-attach.
Used Goop automotive glue to bond the strut to the bumper and let it sit overnight with bricks on the struts to maintain close surface bond.
This glue is a very strong bonding agent and is holding the bumper very well.
Hope this experience may help. -Tom

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'96 C220 138,000mi, '95 E300D 239,000 mi., '87 300TD 214,000mi '88 6.2 Turbo Diesel Chevy Conversion Van 253,000 mi.