100 grit rule

Hey guys,

Just wondering if any of you guys know what the 100 grit rule is when it comes to prepping a car for paint...a painter at a car show was explaining to me something about prepping the car and using the 100 grit rule for sanding and stuff, but i can't really remember what he said...can anyone refresh my memory, or was it just something that maybe he goes by?

thanks8)