On my workbench I have a vise that has a Craftsman 3/8" electric drill "permanently" mounted between the plastic jaw protectors. In the chuck of this drill I have a wire wheel installed. I try to get the ones that have a long shank on them so I have room between the drill chuck and the wheel to work with. I also try to get quality wire wheels (I use the smaller dia. ones, 2-3 inches) that don't have the real stiff bristles.
After I have completely torn down a bike and am in the process of re-building it, I check out the nuts and bolts to see how bad they are. If they're not so badly rusted that they are beyond hope, I will take them to that wire wheel and let it remove the rust. The first items that I tried this on were those rounded-head handlebar bolts. At the time they had not yet been re-pro'd. I was kind of amazed at how well they cleaned up. Yeah, it's tedious work and you need to be wearing eye protection, gloves and probably hearing protection also. Based on doing this on a number of those bolts
I can tell you with
certainty that they have 6 sides and the head is rounded. I have experimented with the wire wheel on other parts also - the shock springs being another one. The wire wheel works well cleaning up rust on them also, if you don't want to go to the work of completely dis-assembling the shock. I slowly turn the shock as I apply it to the wire wheel, it will do more than just the outer surface of the spring, you can clean up the "sides" of the spring also by putting a little extra pressure on the shock as the wheel runs between the springs. I first do one "run" up the shock and flip it over and do the other side.