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. 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.