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By GregShelton
Hi everyone,

I was wondering if there is any equivalence between the number grading system of sandpaper and the F - FFFFF grading of ground Pumice and Rottenstone? For instance, is it possible to say that 600 grit paper equals an FFF or FFFF (... etc.) pumice or rotten stone?

I am looking to fine sand a lacquered finish, then paint on some slightly 3D designs which will then need to be polished, so I will need to move from a flat sanding medium to a powder and I don't know where to make the transition.

Thank you!
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By custard
Good question, I don't know the specific answer but I'll be watching with interest.

From a practical perspective I've used both 3F and 4F pumice in french polishing and I can say that it's easy to both see and even feel the difference between those two grades. In terms of the quality of final gloss you can achieve with rottenstone, from a practical hands-on perspective my impression is it's beyond what can be achieved with the finer grits of wet and dry, and is similar to the medium and higher grits of micro mesh, so say around 6-8,000 grit. But that's just an experientially based stab at equivalence, it'll be interesting if someone has some more objective data.
By Genn

I will take a stab at an answer to this, FFF pumice has a particle size distribution of ~5-75 micron, FFFF pumice has a particle size distribution of ~ 9-14 micron. Wet and dry P600 ~26 micron P2000 ~10 micron.
But this is not the whole picture, pumice comes in ~6 on the Mohs hardness scale whereas silicon carbide and carborundum two common materials found as wet and dry grit come in at 9.5 an 9 respectively. Both silicon carbide and carborundum will not be prone to wearing down very much and whatever fracturing of the particles occurs will still leave a very sharp edge due to their crytalinity. Pumice will on the other hand will dissintegrate and get finer as it is used.

BR Glenn
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By ED65
Greg, I think Genn's post sets you on the right path to finding the approximate equivalents but I'm not sure you really need to know this in practice, because generally you want the powder to be finer than the texture of the surface it's being used on.

Were you planning on painting your designs on to fairly glossy lacquer and then polish up everything to an equivalent gloss? If so I don't think that'll work. If you look at how painted-on details are done on cars, in addition to the decoration being applied to a matt ground (for adhesion) a clearcoat is shot over the top for protection, and it's that which is subsequently polished.

If you go that route you don't need to know the equivalence of sheet abrasive to powdered abrasive, you can use any standard jump such as P2000 and then straight to rottenstone.
By GregShelton
Hi everyone, some really great replies again. Thank you!

In response to your question, ED65, all my research on the subject of Japanese lacquer designs has pointed me in the same direction that you have indicated. Patterns are hand painted on a matt background and then a clear coat is added, which is the one that gets the polishing. That said, I was somewhat confused about my plan so thank you for your answer as it made me clarify my thinking process a great deal.
To give some more detail about the style and process I am hoping to imitate : some designs are deliberately built up to be somewhat 3-dimensional and are often polished and re-polished as the image is built up (as part of the process of creating and shading the image, not applying a finish to it). This rules out the use of sandpaper once the hand painting of the design has commenced (I imagine it won't be able to match the contours of the lacquer design). I had imagined that I would be preparing my matt black lacquer background with a 600-grit sandpaper, so I wanted to know which grade of pumice stone would be similar to this so I can have some uniformity between the black background and the (red and gold) painted designs.
I will be reviewing Genn's great response to get a better idea!
Thank you all!