What is the best product for removing corrosive elements?

I own a Silver 2011 Nissan Altima, and I park it in a New York City garage. I just started parking there, and the first time I picked my car up, there was large areas covered by a substance that was not easily washed off. The garage staff explained that certain areas of the garage are prone to mildew-ey leaks from the ceiling and/or walls. They used a product called "ARS" (Acid Replacement System) which removed it easily. I wanted to purchase this for myself but am finding that it has been discontinued. Is there a specific exterior care product that would work best to remove tough substances that cannot come out in an ordinary wash? I've researched a bunch of different polishes and cleaners, but can't determine what would be the best for my specific situation. Any help would be great appreciated!