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Originally Posted by AngryAlpaca
... I can see my face in my 600 grit lapped WB (I lapped it, it came "Lapped to 1200 grit" but it was crap)... In fact, the features were perfect!
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If you "dry lap" with 600-grit and push down hard, it will wear away the grit and give you a good mirror finish.
"Wet lapping" (I use low-smell kerosene) and low-moderate pressure will give an appearance like I described.
One thing I've been noticing is that the harder one pushes down, the more likely the surface will be non-flat, developing visible rippling which is easily apparant by sitting the block on a table, sitting about 1 meter away, and looking at the reflection of a straight object a few meters away. Move your head slightly and for a surface that has a mirror finish achieved through high-pressure lapping on 600-grit, you will have horribly noticable rippling. My suspicion is that this is caused by frictional heat flexing the metal, which expands and bows out-wards slightly. This "protruding" metal gets hotter 'cos it's now receiving the bulk of the lapping, and so gets worn away more. When you stop, the metal cools and contracts again causing the rippling.
I have had trouble lapping with grits higher than 1200-grit. Using kerosene tends to "suck" the block onto the paper firmly, making it very difficult to move the block about. Dry-lapping tends to generate the "rippling" effect. Lapping at 2000-grit paper requires a LOT of patience, and taking things very, very slowly. When I wet-lap with 2000-grit paper, I end up with a very dull surface. Not even remotely shiny. This, to me, indicates that it is probably fairly flat, as pulling the block off the wetted paper takes quite a deal of effort. If I then follow up with some Brasso/Silvo, the mirror shine comes out very, very quickly, and the reflection test indicates a surface as flat as the human eye is capable of determining. If I dry-lap, I will get a mirror finish from the moment I pull the block off the paper, but the reflection test will indicate rippling.