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CAMI/FEPA Confusion about Sandpaper Grit for Lapping Naniwa 12k

I am planning to lap my new Naniwa 12k and figured I would start lightly with some 600 grit sandpaper on a flat piece of marble I have and then move up to 1000/1200 grit. That was until I asked myself about the prefix "P" before the "600.

So now I know that P means the FEPA standard. So if I want to use silicon carbide sandpaper to mimic Atoma 600 and 1200 diamond lapping plates, do I want P600 and P1200?
 
Don't let the P, CAMI and FEPA numbers bother you, It's not a big deal for what you're doing.

Depending on how flat the stone is from the factory.
If you have one. Put a straight edge on it to see how flat it already is. If not don't worry about it. Pencil hash marks on it, and let them be your guide.
If it's obviously out of wack, start with the 600, or lower, then finish with the 1200.
If it's not obvious, start with the 1200. You'll know pretty quick if you need to go down in grit to remove more bulk.

Rinse the stone off well once your finished. What's going to be left on there is not the type of slurry you want to finish a razor on.
Jmo
 
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Got it! Turns out the answer to my question was easy and is consistent with your guidance, @Dave Sl8r.

I found the grits for the Atoma plates in microns on Workshop Heaven - Fine Tools - https://www.workshopheaven.com/ and then used Conversion Chart Abrasives - Grit Sizes | FINE TOOLS - https://www.fine-tools.com/G10019.html to find the (FEPA) "P" grits for the closest sandpaper and got:

Atoma 400 is 40 microns which is about P360
Atoma 600 is 29 microns which is about P500
Atoma 1200 is 13 microns which is about P1500

Which explains why so many people talk about using P400, P600 and P1200 to lap stones.

So I have a few sheets of P600, so I just need to run to the hardware store and pick up some P1200.
 
The Naniwa 12k is a very soft stone and lapping with grits above 320-400 will have very little effect on smoothness as the stone will quickly smooth out to its natural surface condition after the 320-400.
 
If I need to remove more than a smidge of material from a 12k Nani, I start with 220x. Trying to level that stone with 600x is do-able but exponentially more laborious. I use a 140x Atoma for that work, but fresh 220x w/d will be fine also. I just lapped another 12k Nani a few weeks ago actually.
Don't worry so much about numbers - a worn Atoma 400x is not equivalent to a new one, same for w/d paper. If you start low, it's fine, 320x takes out the 220x striations perfectly. Nothing wrong with using 400x here but it's superfluous if you are going to 600. I don't lap Superstones past that, I usually won't go past 320x w/d or the 400x Atoma; no need unless there is a gouge that can be felt. And that is not a common occurrence. Those stones are designed to wear and release new abrasive particles when you use them; running a blade or whatever over it usually takes care of the marks you see but can't feel and the ones you can almost feel too. Basically, if it is used correctly, the stone's surface will heal itself from 320x w/d lapping on its own 99.999% of the time.
 
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