Coated abrasive sheets

A P prefix denotes FEPA and ISO 6344 grading, while a plain number usually means the US CAMI scale. The scales are close at coarse grits but diverge significantly at fine grits; P1000, for example, is not as fine as CAMI 1000. When changing supply, compare like scales rather than just the number.

Wet-or-dry paper usually has silicon carbide grain on a waterproof paper and resin, used with water or fluid to cut dust, cool the surface and prevent loading on paint, metal or plastic. Dry sheets commonly use aluminium oxide, and many have an anti-loading coating for wood and paint. Do not use dry sheets with water, as the bond and paper break down.

Each step should remove the scratches of the previous one, usually not jumping more than about one and a half to two times the grit number, for example P80, P120, P180, P240. Skipping too far leaves coarse scratches that only show after painting. Wipe off dust between steps so coarse grains do not carry into fine stages.

Open-coat paper has gaps between grains and resists loading on softwood, paint and gummy materials. Closed-coat paper packs more cutting grains per area, suiting metals and hard materials that need an even finish. Choosing the wrong coat type usually means sheets clog early and need constant replacement.

Keep sheets in sealed packaging at about 35 to 50% relative humidity and a stable temperature. Paper backings that absorb moisture curl and tear easily, while overly dry ones become brittle. Store sheets flat, not rolled or folded, until use.

Aluminium oxide is tough, suits metal and wood, and lasts well under pressure. Silicon carbide is harder and sharper but more friable, suiting wet sanding of paint, plastics, stone and non-ferrous metals with a finer finish. Wet-or-dry paper usually carries silicon carbide grain.

The number and position of holes must match the sander pad, otherwise extraction drops sharply. The attachment, hook-and-loop or pressure-sensitive adhesive, must also suit the tool. When ordering, state the size, attachment type and hole pattern.

Replace it when the grain is worn smooth, when the surface is loaded with dust that will not clear, or when more pressure is needed to cut. A dull sheet generates heat, burns wood and leaves swirl marks. Carrying on with a dull sheet usually costs more in time than a new sheet.