Mark, "It was parked in a garage with a concrete roof, which leaked the water dripped onto the body work – leaving some substance that leeched out of the concrete. I have tried acid and alkaline based solvents to get it off but with no luck – any ideas". Just saw the following advert on US TV which might help::thumb: http://flitz.com PASTE METAL POLISH, FIBERGLASS & PAINT RESTORER
Flitz Paste Metal Polish is a concentrated cream that is unequaled in its ability to Clean, Polish, Deoxidize and Protect Brass, Copper, Silverplate, Sterling, Chrome, Stainless Steel, Nickel, Bronze, Solid Gold, Aluminum, Anodized Aluminum, Beryllium, Magnesium, Platinum, Pewter, Factory Hot GunBluing, Painted Surfaces, Glass, Plexiglas®, Fiberglass® and Armatel®.
Removes: Tarnish, Rust, Water Stains, Chalking, Lime Deposits, Heat Discoloration, Lead and Powder Residue, Oxidation, Bugs, Tar, Oil, Fingerprints, Tree Sap, Bird Droppings, Graffiti, Dyes and Black Streaks. Provides Tough, Durable Protection up to 6 Months in Fresh Water and up to 3 Months in Salt Water. Non-Abrasive Formula. Non-Toxic and Non-Flammable. USDA Authorized.
Cheers. The Rock Doc