About I-CAR - Current Events/News: e-newsletters: Advantage Online: July 29, 2002

July 29, 2002 -When discussing grades of sandpaper for a specific repair procedure, the subject is often not completely clear. This is because there are as many as four abrasive grading systems in use in the United States and Canada, and sandpaper manufacturers are not consistent in their use. A recommendation on a repair material instruction sheet to "use 400 grit" is not specific enough. 400 grit in one system has the same scratch pattern as 800 grit in another system. It is helpful to know what the grading numbers mean, and how the different classifications compare to each other on a crossover scale.

Grading Systems

There are three standard grading systems available, each grading the grit particle size to different tolerances:

Coated Abrasives manufacturer’s Institute (CAMI)
Federation of European Producers Association (FEPA)
Japanese Industrial Standard (JIS)

There is also a micron-grade system, but there is no industry-wide standard for measuring micron-grade sandpapers.

Even though the grading systems use different tolerances for sizing the grains, all of the systems use the same methods. Particles of the coarsest grits up to about 220 are graded by sifting the particles through a series of wire mesh screens that have a specific number of openings per inch. The smaller grit sizes are graded through an air-flotation process that separates the particles by weight.

In North America, all sandpaper manufacturers for the automotive aftermarket previously used the CAMI system for all grit ranges. The CAMI system tolerates a wider range of grit sizes than other grading systems and often contains more than one grit size on one sheet of sandpaper. For example, a 200 grit CAMI sandpaper often contains 180 grit as well as 320 grit particles. This is acceptable for the coarser grits, where a finished surface is not the goal, so the CAMI system is still used for grades coarser than 80 grit by most major abrasive manufacturers.

Figure 2–This chart can be used as a crossover between CAMI, FEPA, and JIS grades.

Figure 1–The "P" in front of 240 on the upper sheet indicates a FEPA-graded sandpaper. The words "P GRADE" on the lower sheet makes this classification even more clear.

The European FEPA system uses a tighter tolerance for particle size and is used for the finer grades, 80 grit and higher, by most major abrasive manufacturers. These abrasives are identified by the letter "P" in front of the grit size, such as P120 (see Figure 1). When you look at the chart in Figure 2 it can be seen that CAMI and FEPA grades don't differ much in scratch pattern until grades finer than 180 grit, then they start to differ widely. A CAMI grade of 600 is equivalent to a FEPA grade of 1200 grit. CAMI 1000 is equivalent to FEPA 2500.

Figure 3–Grades 1000 grit and finer without a "P" prefix indicate JIS-graded sandpaper.

Next on the tolerance scale is the Japanese grading system, JIS, which is used by some abrasive manufacturers for grades over 800 grit. JIS-graded paper is identified by a high-grit number without a "P" prefix (see Figure 3).

The highest tolerance is micron grade, identified by the micron symbol (µ) after the number, such as 10µ. Some abrasive manufacturers offer these sandpapers as an option if a highly polished surface is desired.

Wet or dry sandpaper is also graded differently. The major abrasive manufacturers use the CAMI system for grits up to 220, the FEPA system for grits 220–800, and the JIS system for 1000 grit and finer. However, there are small amounts of CAMI-graded wet or dry sandpapers from 220–600 grit, and some FEPA-graded wet or dry sandpapers from P1000–P2500 grit.

Other Factors

With any of these grading systems, all that's being referenced is the size of the grains on the sandpaper. The grading is the main factor and is just about all that a technician needs to know for choosing sandpaper. The other factors are nice to know, and chosen by the abrasive manufacturer to make that sandpaper work the best. These factors include the type of backing material, the weight, the type of abrasive material, and whether the sandpaper is open or closed coat. Any of these may or may not be indicated on the backside.

Sandpaper used for hand and machine sanding for collision repair will have a paper, film, or cloth backing. Cloth backings are more flexible than paper or film backings.

Figure 4–These "A" weighted sandpapers are thin and flexible.

The paper weight will vary depending on the application. Paper weights are from A–F, with ‘A’ being the lightest and most flexible and ‘F’ the heaviest and least flexible. Lower grade, coarse paper like the 40 grit for a sanding board or air file shown in Figure 4 is "E" weight. Fine, wet or dry 1200 grit (JIS) shown in Figure 5 is "A" weight.

Figure 5–These "E" weighted sandpapers are heavy, for use on an inflexible sanding board.

Different types of abrasives include aluminum oxide, silicon carbide, and some product-specific blends. Sometimes, the abrasive type will be identified on the back. Most often, the type of abrasive is indicated by a number and letter code, but these codes are there for the abrasive manufacturer, not the user.

The sandpaper may also say "open coat" or "closed coat" on the back. Open-coated sandpaper has evenly spaced voids between the grain particles, helping reduce the effect of loading caused by the material being sanded. Closed-coat sandpaper has no voids between the grain particles.

Conclusion

Abrasive manufacturers have tried to make the choosing of different grades of sandpaper for different purposes as simple as possible. Most of the information printed on the backside of sandpaper is for use by the abrasive manufacturer. Technicians should, however, know the difference between the grading systems used to classify sandpaper grits. These grades should be recognizable by looking at the number codes on the backside of the sandpaper.

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