Really I think your acid rain in your area would do more harm than a weak 2-step.Now days windsheilds are made more so for acid rain protection.Spraying it on a hot dry surface and letting it dry made burn SS,other than that I think you would be ok.I wash trailers with SS fronts and backs without doing any harm,however I have seen some trailers that look like they have been burnt by some type chemical or letting the chemical dry on a hot surface.Thats what I'm basing my judgement on what caused it to burn.They do make a cheaper SS on some trailers that even a strong saop will burn if applied to it hot and let it dry.

I wouldn't be afraid to try it on the BMW's,this would be a good time to learn flashing if do not already know how.

When In Doubt Think It Out