Grit size is an invaluable metric when finding the right abrasives for your project. Sandpaper that’s too coarse can damage or ruin a surface, while sandpaper that’s too fine will require extra effort and wastes sheets. Very often projects will make use of multiple grits through different stages of the work, so it helps to know what kind of paper you’ll need each step of the way. All of this can be overwhelming, but understanding how sandpaper is graded can help and you may find it interesting, as well.

Grit Size

If you’ve ever looked closely at sandpaper, you’ve seen that it’s made of tiny mineral particles adhered to a backing. These particles are abrasive grains. Larger grains are coarser, and smaller ones have a finer texture. The coarseness of sandpaper is determined by its grit size, the size of its abrasive grains.

A piece of sandpaper needs a uniform grit size, because even minor differences in coarseness can have a huge impact on a surface being sanded. Why is this important? Multiple-sized abrasive grains would smooth out a surface unevenly, impeding your ability to paint, varnish, or do any other finishing or detailing work on it. It may even damage or deface what you’re working on.

Grading

Grading sandpaper helps communicate its coarseness and best uses via a standardized process for measuring grains. It starts with a sifting machine, which uses differently sized filters to sort particles. After the grains are separated out, they pass through a 1-inch filter or 1-inch square in a screen or mesh. The grade is determined by how many can fit through at once. For example, a Medium grade of 100 means that 100 same-sized particles can fit through; an Extra Coarse grade of 24 means that only 24 grains fit.

Grades are categorized into ranges that have different uses, with coarser papers for tougher sanding jobs and earlier surface preparation, and finer grits used on lighter or near-complete work.

• Extra Coarse (FEPA P24-36; CAMI #24-#36): Sanding older floors; removing difficult, heavy layers of paint and varnish
• Very Coarse (FEPA P40-50; CAMI #40-60): First sanding of bare wood floors
• Coarse (FEPA P60-80; CAMI #60-#80): Rough sanding and shaping of wood; removing paint
• Medium (FEPA P100-150; CAMI #100-#150): Shaping wood; sanding of soft wood; removing varnish
• Fine (FEPA P180-220; CAMI #180-#220): Smoothing finishes between coats; final sanding and smoothing of bare wood
• Ultra-Fine (FEPA P400 and up; CAMI #320 and up): Metal sanding; polishing very hard materials such as stone, plastic, and ceramics

FEPA & CAMI Grading Systems

The most common sets of standards for grading ranges are the FEPA (Federation of European Producers of Adhesives) standard and the CAMI (Coated Abrasive Manufacturer’s Institute) standard. CAMI grading is used almost exclusively by U.S. manufacturers, while FEPA is the standard in Europe and various other parts of the world. Sandpaper packaging will generally have both listed, and they’re largely similar until you get into finer grit sizes.

The Golden Rule of Sanding

When you’re deciding on the sandpaper sequence for your project, remember the golden rule: never skip more than one grit! Each successive grit should remove the surface gradually for a smooth, even finish.