The sharper the disc you use the harsher the grind. I use a cutting disc and work it against steel to sharpen it before each use.
As for the tnn clamps the best thing to do (this goes for pretty much any clamp set up really) is to remove the brake and rear wheel, and run a file across the frame's mounts... There's a detailed video of how to do it on the tnn site I believe. You won't regret the effort. Work diagonally accross the mounts with little pressure on the file, hold it in the centre. You will be able to see when all mounts have been filed flat.