Micron size of grit

Does anybody know the micron size of grit on 600g paper? Is it 40 micron?

To further the question, even when lapping there will be valleys in the surface (you can't really make anything perfectly flat). What size would these valleys be? I would assume slightly larger than the size of the part of the grit that did the lapping.

Third question, how much smaller do the particles that make up a thermal compuond need to be than these valleys? Slightly smaller, half the size, or smaller still?

The lapping really has no effect on the microscopic peaks and valleys if that is what you are referencing, they are and always will be there on every surface - flat is always a relative term. The markings left behind from the sandpaper will be slightly larger than the grit, correct. Of course, it makes sense that these markings would be smaller than dents, mill marks, and surface scratches or imperfections which lapping is intended to improve upon.

It makes sense that the particles (since you used that word) that make up a thermal grease need to be smaller than the gaps the grease is intended to fill. The smaller these particles are, the more dense the particles will be, the more surface area of contact there will be, and the better heat will transfer through the material. So the smaller the better.

IMOG said:

The lapping really has no effect on the microscopic peaks and valleys if that is what you are referencing, they are and always will be there on every surface - flat is always a relative term. The markings left behind from the sandpaper will be slightly larger than the grit, correct. Of course, it makes sense that these markings would be smaller than dents, mill marks, and surface scratches or imperfections which lapping is intended to improve upon.

It makes sense that the particles (since you used that word) that make up a thermal grease need to be smaller than the gaps the grease is intended to fill. The smaller these particles are, the more dense the particles will be, the more surface area of contact there will be, and the better heat will transfer through the material. So the smaller the better.

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I don't know about that. I'm not exactly disagreeing with you though. Frome a technical standpoint, you should be able to lap a heatsink past where the thermal interface would be. Pretty much removing it. There will however still be the microscopic valleys left by the lapping process, that you would still need to fill in with a TIM.

Okay, what I've been able to glean from other sources is that 600 grit ranges from 15-30 microns. Would 1/4 micron sized particles be sufficient to get enough surface conact in the grooves left behind by the grit?

I forgot to ask what you were asking for... from your first post, it sounded like you had plans on an experiment with an existing paste or development of a new paste?

@crystal method: Well, I think if one would have the tools to examine accurately enough, he would find that there would be markings from the sandpaper. I don't think it possible to get a perfectly smooth surface with sandpaper or any abrasive milling equipment, right? Perhaps a special laser or something of the sort would leave no markings behind though. Could be wrong.

Ideas for a new paste/compound.