Glad you posted this.

I bought some Norton 320 & 400 wet/dry from Home Despair a while back.
Seems ok, but I'm only involved in very short term projects utilizing wet/dry right now so no real info to offer on how long it lasts.

I couldn't find any P designation on the wet/dry, but I did find a P designation on the Norton 3X 150 and 100 grit dry paper I have in stock.
It's used for woodwork and is lasting quite well in my little Makita finish sander and also when used by hand on hardwood. (Also purchased at Home Despair.)

Since you're sanding dry I wonder if the woodwork oriented Norton 3X would work for you?
Pretty sure it comes in finer grits.

I'm with Nitrowarrior, where the heck did 3M masking tape go?
Doesn't seem to be found most places and auto paint stores here seem to come & go and far as I know the last one faded away a few months back.