Mine looked great when I bought it and I washed every week and used ACF regularly. I bought a bottle especially for the bike. It was early winter. By the spring it was starting to show rust signs on the rear frame.
The bike was mostly standard when I bought it. I fitted a fender extender, hugger then later a Mudsling.
Engine cover finish just seems to be fragile in spite of the mega tough paint that BMW use. Rot first developing from under the belt cover.
I tried stripping back the damaged frame paint and redoing with high zinc paint. The rust was back before the year end. So this summer, I went the whole hog with powder coat and a high zinc undercoat. Rebuilding the FD gave me the push.
For all the moans & groans and the non joined up design thinking the frame is VERY well made. It deserves a proper solid finish with zinc undercoat.