Sandpaper types

I have seen two different 'scales' of sandpaper.

On most of the forums here, it seems like we are talking about 200-400-600-800-1000 grits (someone mentioned 1200 and 2000)

When I went to Lowes, I picked up a few sheets of 120-180-220-320 (I believe there was 380 or something also)

Are these different scales for different methods (Wet vs. dry sandpaper)? or are they the same, and 120-180-220-320 is waaay too rough to be using?

Thanks

A 120 grit paper is waaaay too rough! Unless you are trying to remove old body filler or deep rust, you're just creating problems you'd have to fix later. Home Improvement stores carry sandpaper grits they commonly sell for household projects. What you need to do is locate a paint store for cars, or a good hardware store. Call a few local body shops to see where they get their stuff. I was able to buy a giant roll of self-adhesive 240 grit paper for less than home Depot wanted for a pack of their multipurpose paper.
Heck, I wouldn't even start with 180 grit. I started with 240, then primered and went back over it with 320, then primered again and hit it with 400 before I started painting with the roller. A year later it is still on there and looking good! The reason most of us mention 200-400-600-800-1000 grits is because they work! As proven time after time.

What Coconut said. Also, don't get the orange/yellowish stuff because that is used on wood and is NOT waterproof. You want the black stuff that is waterproof but you can also use it dry. Also get some 1500/2000 for your final sanding before buffing.
Mike

Thanks guys. Good thing I thought of this before I started painting. And I have been using 120 to remove body filler and rust (thanks coconut )