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I had asked this on MIMF some time back and had good feedback but still a little confused. My understanding when comparing the two systems is that P1500, for example, will cut as aggressively as CAMI 800 but leave a "more consistent" scratch pattern. Question one is whether P1500 leaves as fine a scratch pattern as CAMI 1500. Question two would then be why use CAMI paper when "P" will do the job faster. There is a cross reference of the two systems as well as the Micron system on the Homestead Finishing website.
Can anyone add any insight on this subject?
Thanks
Nelson

Nelson, this might help:

Grit Equivalency

Bob--Thanks for the link. I found this link which gives the actual grit micron size along with the cross reference similar to that shown at Homestead Finishing.

http://woodworking.about.com/gi/dynamic/offsite.htm?zi=1/XJ& sdn=woodworking&zu=http%3A%2F%2Fwww.ameritech.net%2Fusers%2F knives%2Fgrits.htm

(How do I add a hyperlink to a message?)

It appears to me that someone unknowingly following 600 grit CAMI paper with P800 grit would be progressing from finer to coarser grit.

It would also seem that suppliers are doing a disservice by mixing CAMI and P grade papers on their shelves. I question whether the "average" craftsman knows that there is a difference in the two grading systems.

Don't mean to be answering my own question here--anyone have any insight?
Nelson

Nelson,

First, I am sure people have made exactly that mistake. I believe that CAMI and P grit are just different scales, in the same way as English and metric units both work for distance and mass. Different ways at getting at the same thing. User beware...

Second, one way to post a link is to click the icon above the message pane that looks like earth with a link of chain, then it walks you through. Your link, for example:

Average grit size in microns

Bob

Bob--Thanks for the hyperlink tip. I was trying to make it difficult by entering the link first.
I've read somewhere along the line that the grit size is controlled more closely in the P grade than the CAMI.
I did compare P1500 and CAMI 1500 under a microscope this evening and the difference is like baseballs alongside pingpong balls.
Nelson

I never knew.

Why am I not surprised that a builder has a microscope lying around.   

I believe that CAMI and FEPA (P-grade) have different tolerances for how "out of spec" they can be, with the FEPA standard being higher than CAMI.

If someone is certain of that, please chime in.

So that's how he gets those purflings mitered.

It would be highly inappropriate to not include in this discussion the question:
What did the dog say when he (or she) sat on the sandpaper?

Let me guess, "Ruff, ruff!"

No, the dog said "Feels like P36!!"

You got me. All I can say is that dog has a sensitive rear end!

There is an article on this very subject in the latest issue of Fine Woodworking.

So are you guys sanding with CAMI or P grade? Do you sand up to a point with P grade and then switch to CAMI?

I can only find P grade up to about 800 around here. Would it be wise to then switch to a CAMI 600 and sand up from there?

_________________
Dave Rector
Rector Guitars

All the papers I've got are P grade, so P grade it is for me..I tend to finish most of my surfaces to 220 grit, using 3M's lovely FreCut gold papers (the stuff StewMac sells), sanded with the grain. Works just great.

Hank--That's interesting, what is the gist of the article? The suspense is killing me. Guess I need to renew that subscription after about 10 years.

Dave--The only P grade I use is P800 and that's because I have a hard time finding 800 in the CAMI grade. I usually sand the final clear coats of nitro with P1500 then go to CAMI 1000, 1200, 1500 & 2000.
I think you're probably right in following P800 with CAMI 600 as P800 is roughly equiv to CAMI 400.
(Whew!--gets a little confusing to me)

Mattia--The highest P grade I can find here in the US is P1500.   Anyone know of a source for finer grits of P?

Really appreciate everyone's comments on this.
Nelson

P2000 papers can be found here (3M, usually, silicone crabide wet-or-dry), but not much above that. I've currently got a bunch of Abralon discs I'll be testing as finish 'polish' on my next instruments, pre-buffing. Level by hand with 600 or 800 grit, then go to the Abralon pads. I would think places like Homestead Finishing and Target would carry higher grit papers, ditto someone like Supergrit (they've got 2000/2500 P graded papers in the catalog I've got of theirs)

Mattia
I'm not seeing any P grade paper at Supergrit.
I've looked for P grade in the finer grits at Homestead also but I believe P800 is as fine as they go.
Am I looking in the wrong places?
Thanks
Nelson P.S. Thanks for mentioning Supergrit. I had not heard of them and their prices are good.

Hmm...give SuperGrit a call (ditto Homestead) and just ask them if they have any would be my advice. I got several things from them that they didn't seem to have in their online catalog, but did have in stock. Besides, if they don't stock it, they might be able to tell you who does!

The jist of the Fine Woodworking article is that FEPA paper is "rougher" but more "uniform" in grit size. Example...a paper rated at 1200 grit on the FEPA scale is about equivalent to a 600 grit on the CAMI scale, but the particle sizes are more uniform, resulting in a more consistent scratch pattern. In the coarser grades, they are about equivalent in particle size, but the difference really starts to be noticeable around 320 grit and up.
For really fine abrasives, check out the Micro Mesh line. They are a cloth backed abrasive system, using the FEPA rating and go up to 12000 on the grading system. I'd guess that would be somewhere in the 5000-6000 range in CAMI, if there were any papers available at that grit....I've never run across any.

PS: this issue of Fine Woodworking also has a nice discussion on the 6 essential glues and their appropriate uses.

I have been using Micro-mesh for some time now and can't picture working finishes with out it. You can get a very good finish with just polishing after the 12000 FEPA. It is expensive but last for me about 10 times as long as paper backed especially when used wet, and produce less heat. Most people are not aware that it is available in large sheets and pre-cut hook and loop pads for your ROS with holes or without. I get it without holes and punch my own with a leather hole punch. See www.micro-surfaces.com

Do you guys start with Micro-mesh 1500 when doing the final sanding on nitro?
Thanks
Nelson

I use 1500 - 3200 on my final leveling prior to cure, but on final sanding after it has cured, I start with 2400 - 3200 - 4000 - 6000 - 8000 - 12000

Thanks, everyone.
Your help is greatly appeciated.
Nelson