The grit size of sandpaper determines how coarse or fine you sand. The number — the P-value — indicates the fineness: the higher the number, the finer the grit. Here is a quick overview for the most common applications.
Coarse sanding (P40–P80)
Used for removing old paint, rust or major surface irregularities. Cuts aggressively and leaves visible scratches that are worked out in the next step. Suitable as a base step for metal, wood and bodywork.
Medium (P100–P180)
The workhorse grits. Ideal for flat-sanding filler layers, working up primer and preparing the surface for the final coat. In this range you remove the coarse scratches.
Fine (P220–P400)
For interlayer-sanding of paint coats, refining the substrate and removing dust nibs from the paint. On wood, P220 gives a smooth final result for oil or stain.
Very fine (P500 and higher)
Wet-sanding grits. Used for removing orange peel, runs and overspray from the final coat, and as preparation for buffing and polishing. Always use wet for best results.
Rule of thumb
Never skip more than two steps. Jumping straight from P80 to P320 costs final quality — the finer grit then has too much work to do. Always work step by step.