Sandpaper and a board. Seriously- you want to lap the surface down down to metal.
If you know what lapping means, you're all good to go. If you don't, read on.
Lapping means sanding only the high points. The real trick with gaskets is that you want no nicks in the metal- that's a killer, because it lets pressurized oil start seeping through. Bad, bad, bad. The goal is to end up with a totally flat and even surface. Now, if you just plain ol' sand it, you risk creating high points and low points- very bad. BUT if you lap it, you're only taking off the high points.
For any gasket surface that's flat, it's easy- get a flat board (or, better yet, a piece of glass), lay some 250-300 grit sandpaper on it, and gently rub your sealing surface and your flat piece of sandpaper together. Doesn't matter which one you move, as long as you have more or less even pressure. Since the gasket material sticks out, it's the first to go.
For the base gasket, which has those annoying studs coming out of the surface, I used the side of a board. Specifically, I had a board that was 1 inch thick, 6 inche wide, and about 16 inches long, with nice, crisp corners. I folded the paper over the edge, and then used the edge of the board to lap between the studs. It's not ideal (you're not lapping the entire surface at once), but as long as you make sure you're changing directly often and have as much surface contact as possible, it's fine. You still might need to clean up some areas inside the studs with a razor blade afterward, but it shouldn't be hard.
An old race mechanic who used to work on these bikes back in the 70's taught me this trick- I had all the surfaces cleaned up and perfectly smooth in just a couple of hours. Be careful, go slow, and focus on making it flat more than cleaning off a given speck, and you'll be in great shape in no time.
Oh, yeah- and have a beer while you're at it
Have some pics!
Head, before:
Head and jugs, after:
Base, after:
