Slash McCoy
I freehand dog rockets
Sandpaper, glued carefully to a 12 x 12 polished marble floor tile or a granite countertop sink cutout or a 12 x 12 plate of 3/4" acrylic or something else at least as flat as these. Don't use "flattening stones" which themselves need to be flat, and which are no bigger than the stone being lapped in most cases. You want to minimize overrun for best results. The sandpaper must be applied tight and smooth, no lint or dust or hair or bubbles under it. I use usually 220 to 320 grit wet/dry initially and then when the grid is nearly gone, I move up to 600 grit. Then I hone a stainless steel chef knife on the newly lapped stone to break down the surface a bit. This is optional with synthetics. Mandatory with arkies or other super hard naturals.Slash, that was a great response. I have all the info I need, thanks to you. Except one question: What should I use to lap my stones?
Some guys use glass and abrasive powders. The glass needs to be resting on a flat surface because it flexes. And the glass needs to be replaced often. However it does work. For me, sandpaper is just too simple and convenient.
There is lapping for cleaning and restoring the surface, and there is lapping to ensure a high degree of flatness. The former can be accomplished by rubbing the stone against another stone of not too dissimilar grit, under running water. If you stone loads up with swarf, for instance, you can give it a brisk rub on the next finer or coarser one and carry on as before. The Naniwa SuperStone 3k is notorious for loading up with swarf, and I often give it a rub on the 1k or the 8k between razors. Some guys like to use three stones in a round robin scheme to flatten, but it won't work as well as the sandpaper or the glass and powder.
You can also get a calibrated and certified granite lapping plate, and these are not as expensive as you would expect. Top the plate with sandpaper or lapping film to flatten most anything to a very high degree of flatness.