Sorry guys, been away from the computer for awhile. Wow, I don't really post much, but it looks like my photos have stirred some attention.
First off, I would like to credit TFL member Dhart from inspiring me to do this. It can be a little nerve-wracking to put sandpaper to a new revolver.
Here's what I did. My method was a little different from his, but it worked for me. First, I took an electric drill with a 5" adhesive backed sanding disk with 120 grit disc. This was foolish of me. This was way too big for this project, but it was all I had available at the moment, and I can be impatient. I tried to be really, really, really careful.
I would recommended a dremel tool with a 120 grit or so sandpaper drum. Brush over the billboard lightly, trying to maintain even pressure as you move it along the length of the barrel. You want to maintain consistency in the radius of the barrel, rather than only grinding away the billboard, which will leave a noticeable concave surface (which if you look closely, that is what happened to me. I fixed that, though.)
After I removed enough metal to be done with the billboard, what I did was take a small 2" or so length of automotive heater hose, cut it like a pie lengthwise until it matched the curvature of the barrel, and backed that with a small piece of wood. This was my sanding block. I used 50 grit sandpaper and went back and forth, always trying to maintain even pressure and sanding in a straight line, for about an hour until I could see that the barrel was about as straight as I could get it. This levels it out so you don't get a bunch of "ripples" in the finished product.
Anything else that I didn't take the drill to, I just sanded without a block; that metal was already straight and my sanding didnt seem to change that. I progressed up in grits, next to 100, then 150, 220, 320, 400, 600, 1000, and finally 1500. I say once you get up around 150, you've done all the leveling you're going to do. 150 grit doesn't remove much metal; it just removes the larger sanding scratches from the coarser 100 grit. From 150 grit on, I just used my fingers (no block.)
After doing all this up to 1500 grit, I brushed everything with a red Scotchbrite pad, and then spent about 30 minutes or an hour or so polishing with Mother's Mag and Aluminum polish (the stuff in the metal can, not the liquid in the bottle.) I've heard of others using Flitz, too.
So that is the basic process. The main thing to keep in mind is to not be in a hurry. Don't move on to the next finer sandpaper until you've ready. Don't just work on the barrel, which is easy. Pay attention to the small crevices and smaller areas that are harder to work on. Finally, spray everything down with cleaner before reassembling.
Oh, last thing, is be prepared to explain what it is to the guy at the range who asks.
Here is a better shot of the finished project. Notice the concave area where the billboard used to be is reduced. Cheers, everyone.