OK, forgot to answer with my 'fix'.
I got a long shaft 3/8 drive and took off the 2 bolts holding the fuel door.
Real simple, except the spring needs to be wiggled to get the socket seated/in firmly.
Then poured a little water down the drain hole to test. Working fine.
Then used several types of cleaners, but some old 3-M stuff used to remove scuffs/mars/bug/tar finally worked. (Took the spring out to get in the cracks.) Used an old toothbrush (careful, that can melt!)
Once done, I got out my can of good old paste wax. Swirled it all around and one the clean backside of the door. Once done, got rid of excess, then simply re-bolted the door to the car. Took a few tries to get perfect alignment.

Done ! A success story !!!!!!!

__________________
BnObob (for the fallen flag railroad.)
Artic "Snow" White 1989 300E, 124.030
w/ M103 engine, and 106K orig miles.