My Volk SF Challenge would develop hard to remove 'speckles'. As these speckles got worse and worse with age i realised what was going on. Small stones/grit gets inside the deep dishes and bounces around, putting very small chips and punctures into the lacquer. Salt, dirt, brake dust etc gets into the small punctures and gets under the lacquer (well it's not a lacquer on Volks in a form of ceramic coating) and you end up with a right ol' mess - long story short, i'm trying to strip off the anodising and repolish my lips because they were ruined!
Do yourself a favour and only use your Volks in the summer!