There are a few different types of grit that can be used to block sand after primer. The most common type is P180 grit. This grit is typically used for sanding metal surfaces. Other grits that can be used include P220 grit and P320 grit.
P180 grit is the best choice for most applications. It is a versatile grit that can be used on many different types of surfaces. P220 grit is a finer grit and is best used for sanding painted surfaces. P320 grit is the finest grit and is best used for sanding delicate surfaces.