by Walter Hinto
Sandpaper is sold in sheets or in packets ill small squares. Buying by the sheet is more economical and allows greater flexibility in shaping pieces for specific jobs. Sandpaper consists of a cloth or piper backing and an abrasive mineral coating. To get the best sanding results, wake sure you select a sandpaper with lime right combination of mineral, backing, grain spacing, and grain size. Minerals: Flint paper is the cheapest lull it cuts slowly and dulls rapidly. Because of its low price, it is a suitable choice for the initial sanding of softwood surfaces that clog paper quickly.
Flint paper is available only with paper backing and closed grain spacing. Garnet paper is durable and is used for smoothing, finishing, and polishing lime surface. It is more expensive than Hint, but it is the best choice for general sanding. Silicon carbide is excellent for extensive sanding, but it is more expensive than garnet. It wears well; wood dust can easily be knocked out of it, allowing the same piece to be used many times. Aluminum oxide papers are most often used in finishing. Backings: Coarse sandpapers used for rough carpentry jobs have heavy backings for durability.
Fine sandpapers used for finishing have thin, soft backings for flexibility and to reduce scratching. Backings come in three weights: A, C, and D. A is the lightest. Abrasive spacing: Depending on the spacing between grains, sandpaper, is classified as either open or closed coat. Closed-coat grains completely cover the backing; open-coat grains cover as little as 50 percent of the total surface area. Open-coat papers cut faster with less pressure.
They last longer because they are less likely to clog up, and if they do the dust can easily be shaken out. They are therefore especially suited for use on softwoods and painted surfaces, which clog sandpaper quickly. Aluminum oxide paper is also made with a zinc-stearate coating to reduce clogging. Grade, or grit: Grade, or grit, numbers refer to
the size of the mineral grains. The higher the number, the finer the grains. (See chart at right.)
Use the finest grit for the job at hand; for furniture repairs, medium should he the coarsest you will ever need. A rule of thumb is to take more time sanding with finer grades rather than risk damaging a surface with a coarse paper. For small areas, use only one fine grade. For large areas, particularly those to lie refinished, start 1 with medium, and go on to liner grades.
Sanding technique: Use straight strokes with a light, even pressure. Move the paper back and fort h in a continuous motion. Work over large areas and be sure to sand all parts equally. Always sand with the grain, if possible; sanding across the grain will scratch the surface. On curved areas, sand first in the direction of the curves, but sand the final strokes with the grain. On flat surfaces and edges, use a sanding block or some form of backing other than your hand to avoid producing wavy contours.
Types of sandpaper
Medium: Coarser papers are rarely necessary for furniture repairs. Except for removing heavy old finishes, you should not require a grade below No. 80.
Fine: Grit numbers range from 100 to 180. These grades are used for the final sanding before you apply fillers or paint. Usual A-weight backing provides conform ability on curved surfaces. Heavier C-weight backing is best for flat surfaces.
Very fine: Grit numbers range from 220 to 280. Papers have an A weight backing. These grades are used for sanding between applications of finishing coats. No. 220 is suitable for the final sanding before the refinishing of softwoods; Nos. 220 to 280 are suitable for this same purpose on hardwoods, depending on the hardness of the particular wood.
Extra line, or superfine: Grit numbers range upward from 280. Finest (highest numbered) are used for the final polishing when finishing hardwoods. They produce mirror like finishes.