If at all possible, avoid using any stripper containing Methyl Chloride. It is a really bad chemical, both for disposal and your health. It will cause chemical burns on your skin and various organs are affected by the fumes. There are really good reasons it has been banned in the industry.
The peroxide based strippers on the other hand are Ph neutral, so it will not harm the metal. If you get it on your skin, it just bleaches it. It smells funky, but doesn't cause organ failure. You do not need to cover it with plastic to keep the volatiles in (the stuff that harms you anyway). It is kind of a slime and stays on without evaporating. We leave it on for 24 hours and it usually pops the paint off. Old enamels that are very oxidized turn into a goo. By far they are the best strippers we have used in a long line. They get multiple coats off at once and require less labor to clean up. Best of all, our disposal fees are way down.
If you alodine, use plastic under it as well. It is also considered toxic and hazardous. Prep the metal well if you use it, let the runoff dry on the plastic and wrap that up and dispose of it in a plastic bag. The metal should be warm when you apply it, or it really will not react well with the surface.