Anything below 240 should never touch the edge tools! 60 is good for floor sanders. Have you ever seen 60 grit sharpening stone? 240-400 is what to be considered as coarse for sharpening. 400-600 medium and above 800 fine or super fine. Good quality 240 is very effective on hard metals, doesn't have to be wet and dry.
Not particularly common since 'open coat' papers are generally used in the lower grades, especially regarding car bodywork and general roughing out and shaping. These grades are used dry as you don't want to introduce any moisture when shaping filler etc. A tad off topic possibly, but 'open coat' grades don't necessarily tally with wet and dry grades if my memory serves me! Anyway...just sharpen it. I think people get too an*l when it comes to putting an edge on a chisel...as Chippie mentioned early on. Sharpen it...and get the job done
If its polishing his chisels then it must be 600 as HA says. A photograph of the chisel and sandpaper would settle it.
I sharpen my everyday chisels and irons on a little bench grinder and polish them up with soap on a buffing wheel. I don't bother with angles or lapping or anything. A Zimbawean guy showed me and it produces crazy sharp results with very little time taken. I wouldn't do it on my best ones but working ones no problem.
Just letting it cool a bit while grinding sorts that. I have been told that dunking hot chisels in cold water is very bad for them. Microfractures the steel apparently.
Just on this topic over the years I have found there really are two kinds of sharpening that get done. Sharpening for show and sharpening for go. Seen guys spend all day sharpening up a set and produce garbage work and I have seen people take minutes on a set and produce great work. It doesn't matter to me how they get sharp as long as they are sharp.