I think I made a slight mistake IIRC in that I used the sandpaper on the body of the Lee Auto Disc measure when I converted it to the double disc setup. I placed the paper flat on a piece of glass which was supported on a doubled over bath towel on the kitchen counter top. Then simply rubbed it back and forth a number of times, keeping it perfectly flat to the paper and checking often so that I didn't take too much off, until it mated with the sliding disc perfectly. When I say perfectly, I mean there was absolutely no light to be seen between the body of the measure and the top of the sliding disc, but the disc still slid back and forth with no effort.
Now to the Perfect powder measure. I could never find a really good way to use sand paper due to the crazy angle of the drum. So, going back to some of my old race car engine building days/techniques, I removed the drum from the body, removed the cut-off wiper from the body, then leaving all the other assorted parts/pieces off the body, I added just a small amount of auto valve grinding compound/grit to the inside of the body, all the way around. Inserted the drum making certain that the adjustable thimble is pulled back inside the drum far enough to not be hurt while grinding. By keeping just a small amount of pressure with my hand/fingers, I continually rotated the drum inside the body back and forth which I held in the palm of the opposite hand. Every so often I'd remove the drum, wipe it and the body down real good and see how the match marks were doing, then reinsert the drum at a different point of rotation than the last time, and after painting the entire inside of the body with IIRC yellow high lighter. The stuff you'd use to mark/highlight certain verbiage in a letter for example. Rotate some number of times, remove, wipe and repeat till all the highlighter is gone if needed. Keep in mind the drum will wear away faster than the body so watch it closely and watch for match marks all the way around. I'm not certain about the highlighter, it may have been a Sharpie pen. The reason I'm saying that is it seems like I was using something red far more opaque than highlighter by the time I was done, but can't remember why. And coloring, even Dykem Blue would work I'm sure. All you're looking for is something to tell you if you have 100% mating surfaces and if you know what to look for you can see it in the grinding scratches on the parts without color. But I'll tell you this, once you've done this, you'll think a whole lot more of the little measure. By the time I finished mine, I could run tons of "True Blue" powder through it and never lose a grain. Well, maybe not a ton! You can even get an adapter on Ebay, etc, to mount the little measure directly to the top of the powder through expander die when loading if you want. It's much better and faster than dumping from the measure into a pan then into the case, or even removing the resized case and holding it under the measure. Give it a try, you'll like it. BTW, I haven't bought any in a long time but used to buy valve grinding compound at most any parts house in a small container just a little bigger than an "Alka Seltzer" tablet for a couple dollars. Be aware, that was probably 40+ years ago I bought the last tin I had and used on the powder measure a few years ago. Let me know if there are more questions.