Boy, have I got the answer (or at least A answer) for you. I had the
same problem on ten-year-old biothane that looked gross. During the
latest rainstorm, I got out the Dremel (sort of a lightweight handheld
drill, handy for billions of things) and used the polishing wheel
attachment on the buckles and rings with a bit of silver polish---it
shined up GREAT!!! Also took alot less time and got better results than
trying to scrub all that gunk off by hand.
Just don't use too much silver polish, cuz I found out as soon as you
turn on the Dremel, it flings it all over the walls (VBG)!!
Good luck!
Susan