help understanding sandpaper "P" grade

[projectPONY]

been out of the loop for a while and I'm confused about this "P" grade system for the automotive abrasives. I don't know how long this sys. has been out but I came out of paint and body school back in '98 and I remember the grits being just plain jane 180, 220, 400, and so on. now, I make a trip to the paint shop and the guy behind the counter asks me if I need 400 or P400 and to top it all off he didn't know the difference:smoke:. what gives? are they not the same grit? what is the difference in them, is one finer or coarser?

thanks,

Carlos

[wiedemab]

Go a about half way down the page on wikipedia's entry for sandpaper.

http://en.wikipedia.org/wiki/Sandpaper

The P is a different grit designation. It looks like P400 is more coarse than 400. You can compare the particle size in the chart.

[projectPONY]

first off, thanks for the link.

ok, so P400 is coarser than 400, so is it too coarse to prep primer with before base coat? seems like P600 would be a safer bet.