Jordy,
PITA = pain in the ass, not a type of bread.
Usual caveat on the Milton, it worked for me, but use at your own risk. I diluted the Milton 4:1 in warm water, brushed it on an area with a soft bristle car wash brush, watched until the black spots faded which took about 5 minutes and then rinsed off with a hose. Repeated until all the skins were done and then thoroughly rinsed the whole thing CAREFULLY using a pressure washer with the nozzle set for a wide fan, not a concrete cutting jet, and held well away from the surface. Let the breeze and sun dry it and make sure no water trapped anywhere, re-lubricate any moving parts or hinges before flying it. As I said, no problems or significant fabric deterioration in 3 years afterwards.
If you search on the Xair forum there was a long thread about it at the time and my photos are filed.
Nick