Stripper works for me. I use Jasco from the hardware store. I apply it with an acid brush and wipe it off with paper towels or newspaper, then I go after it with a wire brush. Some spots may require a second or third application. I wash down the frame with lacquer thinner when I'm done. I took the original paint off of a '74 Raleigh Competition a couple of weeks ago in about 45 minutes. I've never had stripper cause a problem with the paint.
The stripper burns when you get it on your skin, but it won't even leave a mark if you wash it off right away. I got some on my eyelid once and that hurt! I always wear safety goggles now.
I have no experience at all with aluminum frames and I probably never will.