Sometimes I'll use preformed cork grips and modify them by turning them on my Pac Bay wrapper, using whatever sandpaper I can find at the home improvement warehouse. I've noticed that I often get dark splotches on the cork. By dark, I mean somewhere between black and gray, almost a charcoal color. The splotches are mostly round, in clusters, and only in certain places on the grip. Is this from ratty cork, bad sandpaper, or poor turning technique? I believe the paper I use is Alum Oxide, up until the 400 grit, which I can only find in wet/dry but the splotches are already there by that time.

Greg,
You may have have the splotches in the cork, or you may be putting the splotches there by the sand paper that you are using.

Generally speaking when turning, you should be using fresh sandpaper, and you should not be using heavy pressure.

If you use fresh - reasonable quality -- sand paper and light pressure when sanding, you will not be leaving splotches from your sanding technique.

There is no question that much of today's cork is of a much lesser quality than cork of years ago.
Simply put, there is less quality cork in the world today due to the use of cork in hundreds of different products and the finite amount of cork that can be produced from the finite amount of cork trees in the world.

One thing that can be a clue is to look at the cork rings when you receive them. If there are signs of discoloration in the sides of the cork rings, it is likely that this discoloration will show on the surface of the rings when they are glued and sanded down.

Another thing to try, if you happen to have some other scrap rings, is to glue a bunch of them together, and then; rather than turning them down, take a saw and cut the rings in half. If, when the rings are sawed apart, they show the discoloration that you are seeing in the turned rings; the issue is in the cork. But on the other hand, if you see no signs of discoloration when the rings are cut apart with a saw, then it is more likely that the sandpaper and sanding pressure that you are using to turn down the rings is causing your problem.

Generally when turning rings, I start with a Stanley surform cutter to do all of the initial shaping, to cut through the glue line and to cut the handles down to about the 80% point.
Then, I will move to 100 grit paper to get about the 85% point on the grips.
Then, I go to 150 grit for the next 5%. Then, I will go rather quickly through 200, 220, 320 and generally finish with 400 grit paper.

By the time that you get to 400 grit, the paper is not cutting very much. Rather it is doing more of a polish. I have found that with cork and its texture, that using paper finer than 400 is generally a waste of time and can possibly end up burning the cork due to the fine nature of the paper.

Good luck and be safe
Roger

There are certain types of fungus that can occur in cork. Normally, bleaching the cork during the preparatory process kills any such fungus, but with pre-formed cork it's hard to tell what you really have there. Not all cork is bleached.

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I have seen new rings with a blue/gray pigment on occasion. these are small to medium spots in the ring but aren't consistently through the ring (hope that makes sense). If it were your sanding technique, the rings would probably look uniformly dark. I have this effect when I inlay burnt cork with regular cork. It just gets dirty. A good cleaning with alcohol brings back the bright color of the regular cork.

Terry