^This ^This ^This

Works like a charm. In the video, the BB shell in question is the PERFECT candidate for the wire brush technique. I've used this technique for many years and it's the way to go. His threads still had residual paint on them as well; the wire brush will remove pretty much everything. And one more thing: the wire brush to get has a larger OD than the BB ID, so it takes some effort to get the bristles into the BB shell. This is good as they're forced as deeply as possible into the threads. Don't get one so small it isn't a tight fit.

Now as far as the home-made thread chaser goes.... Looks like a cool project. However, note that the tool you create (and hopefully from a set of steel BB cups, not Al) will not be actual tool steel, so it will dull quickly and not cut very well. Also, his cuts with the ceramic wheel seemed to leave the slot somewhat rounded. If I were cutting, I'd want the edges either vertical, or even undercut a bit so the tool can pull material away instead of just mash. This is how taps are cut in many cases. Oh, and use oil when you're using it, preferably cutting oil.