Wide (as opposed to Sharpie pointed) magic markers (permanent ink) are less messy than prussian blue. The low spots retain the color and the shiney spots are "high".
Another way to lap is to use progressively finer grades of wet/dry sand paper (220 to 280 to 320 to 400 to 600 etc) on a piece of thick (1/2" or 3/4" glass). Keep the paper wet (water is fine, add a couple drops of dish washing detergent as a wetting agent).
Don't just lap in 1 direction. By changing directions, you can see the low spots by the sand marks that did NOT disappear after you changed direction.
We have generated near mirror finishes with this method if you get finer and finer - it is the same method used when you do metallurgical grain structure analysis so it is not some sage wisdome deal.
Can be tough to lap a milling table this way but the project you're working on will work just fine this way....
EDIT: files tend to cut in one direction. Thus you do NOT want to slide them back and forth under pressure. IT is pretty easy to "see" which direction the file wants to go to cut.
So, push in proper direction to cut, lift and move file back, re apply to surface and repeat. Hans will probably have a royal fit if he sees you sliding it back and forth as if it were sanding block.  |