been an ongoing problem
touratech who make them have admitted to using a thinner / lower grade aluminium to the last ones for the GS TC and earlier models, BMW know about this and some have been replaced due to this, mine however did not get replaced and the two panniers have white dots all over them and I never even used them, bike got delivered in a van, the boxes taken off and into the wardrobe, gone off in two days
the only thing apart from above is to polish them with autosolve and get them as good as possible then coat them in acf 50 / gt85 but not wd40 as it just runs off and doesn't last as long