Your best bet with corrosion like that is to first flatten the metal with varying grits of sandpaper. A 5 inch orbital will do. Start with 40 grit, then step up the grain count with a sequence of something like 100, then 200, 400, 800, 1000, 1500, and finally 2000. By the time it gets to 1500 it will start looking like a mirror. It's time consuming, but the results speak for themselves. Only then is polish going to work effectively. See the difference in the attached pic from the section un-sanded to the section sanded (this is before polish). As for the compound itself, I've had nothing but impressive results with Gords liquid polish. Apply with a regular terry cotton rag and buff it away with a microfiber. You just keep applying it in stages until the cotton rag no longer hazes black. That's how you know when it can't be polished any further. As a final step, a clean rag dipped in acetone takes away all traces of polishing residue. At this point you'll be able to shave your face looking at the pipe and can be so incredibly bright that sun reflections can be blinding to onlookers.