Lapping Question

Used to have a decent watercooling setup... then I got out of overclocking for a short while, and stored the WB and such. Found the WB again today, but from the storage it looked like it had a few small dings and scrapes in the bottom, so I took it downstairs for a lapping session.
This is a DD Maze4 block. What scared me is that, when lapping, the block began to develop a ring of smooth material along the edge of the block and a progressively rougher look towards the center, which suggests that the block is starting to get a concave shape. I had run in to this problem when lapping ramsinks, but didn't figure on having the same issues with a CPU block, since it's so much larger and less likely to "roll."
So, I can only think of two explanations for the concave effect- that the block was shipped with a somewhat convex bottom (which is unlikely at best with DD), or the block is "rolling" as I sand it. I was making a conscious effort to prevent this, and I noticed no movement- the block felt like it was moving perfectly parallel with the sandpaper the whole time. The corners do appear to be somewhat rolled up, but the rest of the block looks flat.
Which of these is likely my case? Is it normal to have a slightly uneven block? Or, is it a common mistake to have the rolling problem? Any suggestions?

Any help appreciated

I'd run into this problem with some Aluminum P4 coolers, most concave in the center, and you would only seem to lap the outer corners, leaving the inside scratched from the uneven touch. Is this what you have encountered also?

I doubt that your sanding it made it convex. I suspect while in storage it may have warped or distorted in some way. Just a hunch though, and I'm unfortunately not educated fully in w/c or blocks. I imagine the best way to remedy this is to mark the bottom with a sharpie a series of X's and keep sanding, waiting until you can make an X and sand it off evenly (Hopefully this makes sense?).


WELCOME TO THE FORUMS!
and a bump for your post!

Its most likley that it just had a concave base to start with

Try reading some of the 1000s of lapping related posts on these forums to check you are doing it right. If you are then just keep going untill its flat.

Maybe start with a corser grade of paper, say 320 then 600 then 800

In lapping I first used 220 grit (the closest I had to a "first run" grit) and it seemed to sand evenly. So I moved on to 400 grit, and that's where the problem seemed to be ocurring. And yes, I have seen this problem with aluminum ATI heatsinks (the only other thing I've tried lapping), but those seem far more likely to me to have an uneven base. Also, good idea with the sharpie ffats, I think I will try that.