Meguires I agree. Now if the staining looks too bad then you might have to have new vinal put on. I asked the guy at the Boat Tops and More shop and he said the only way to prevent it is to keep it out of the sun and my reply was that won't be possible for a moored boat.
My top is now four years old and it looked like some thing that acid would take off. I used phosphoric acid and it did not work, it was way beyond polish so off to the repair shop I went.
Second question - Acid wash will take off the Oxidation, but this oxidation is what will protect the alluminum hence alluminum oxide a very hard substance. It won't look nice and shiny but it will be protected.