Know How: Sandpapers and Sanding

Although the technique of using sand as an abrasive goes back to the stone age, the first recorded example comes from the 13th century, when Chinese craftsmen bonded sand, crushed shells and sharp seeds onto parchment with natural gum. In the western world, abrasive paper was being used in France by the 18th century and “glasspaper,” which consisted of glass particles glued to paper, was mass-produced In England by the 1830s. The U.S. Patent Office issued a patent for sandpaper at about the same time.

Today those shells, seeds, glass and even sand have been largely replaced with synthetic abrasives, primarily aluminum oxide and silicon carbide, and sometimes by the minerals garnet and emery. Emery, which is black, is an impure granular form of corundum, a mineral that, in its pure form, is second in hardness only to diamonds. When it is crushed, emery isn’t very sharp, which makes it better for cleaning and polishing metal surfaces than for conventional sanding projects.

Garnet is sharp, but not very hard, which means it wears out quickly on a hard surface. Garnet paper has a distinctive reddish color; woodworkers love it because it produces a very smooth surface finish and tends to burnish wood. This is desirable when using pigmented stains. But neither garnet nor emery are good for most sanding jobs on a boat.

You might also come across flint paper—actually it’s quartz—and typically its color is light gray or beige. Flint paper is cheap, but you get what you pay for; quartz is neither hard nor sharp.

Aluminum oxide is the most common sandpaper abrasive. Fused aluminum oxide is essentially synthetic corundum, but is purer than emery. Because it is brittle, aluminum oxide paper creates fresh cutting edges while the paper is being used. That’s why it tends to last longer than other sandpapers. Typically tan or brown in color, aluminum oxide paper is the best choice for most marine refinishing projects.

The basic rules for what sandpaper to use, and when, are not complicated. Dry sand with aluminum oxide. Wet sand with silicon carbide. Bottom sand with a sanding sponge.

Silicon carbide, or carborundum, has a black surface and is commonly known as “wet or dry” sandpaper. “Wet or dry” is a trademark of 3M, the company that originated waterproof sandpaper. There is also open-coat silicon carbide paper, which is white. Because silicon carbide is harder than aluminum oxide, it’s the best paper to use when sanding hard materials like fiberglass, paint and metal. It’s not good for sanding most woods or other soft surfaces.

What paper to use?

For wood and most of the other surfaces on a boat, the best choice, even for power sanding, is aluminum oxide. For wet sanding, metal sanding, between-coat sanding (or if you’re involved in a gelcoat restoration), use waterproof silicon carbide paper instead.

Grit counts

Every piece of sandpaper has a grit number that describes the particles of abrasive per square inch of paper. The higher the number, the smaller the particles and the finer the finish you can expect. Grit numbers run from 36 to 1000, although there are some papers that exceed this range at either end. Cheaper papers are labeled coarse, medium and fine, which generally translates to 50-, 100- and 150-grit paper. Whenever possible, avoid sandpapers that do not have a numeric grit designation.
Most grit numbers also have a letter describing the weight of the backing paper; the most common designations are A, C, D and E. A is the lightest and least durable. When you have a choice, use paper designated C or D.

Open and closed

The backing should also say whether the paper is open or closed coat. Closed coat is more common and this means that the entire paper surface is covered with abrasive. Open-coat paper has about 40 percent of the paper covered with grit, so even though the grit is the same size, there is less of it on the sheet. The extra space between the grit provides room for sanding dust, which makes open-coat paper less prone to loading, where sanding particles become trapped in the paper. Open-coat paper is a good choice when you’re sanding soft wood, stripping paint or varnish, or using a high-speed power sander.

Closed-coat paper cuts faster and more smoothly, which makes it the best choice for most other sanding projects. In most cases it makes sense to start with closed-coat paper and switch to an open-coat paper if loading becomes a problem.

One important note: The backing for regular aluminum-oxide sandpaper is not waterproof, which means moisture can ruin the paper. Silicon-carbide paper does have waterproof adhesive and backing, so it’s good for wet or dry applications.

Whenever possible, buy sandpaper by the sleeve (25, 50 or 100 sheets, depending on the grit), because it is much cheaper; the per-sheet price can be half of what you pay for an individual sheet. However, a sleeve of sandpaper can last a long time, so it’s best to store it a moisture-proof box.

Sanding tips

To get a uniform finish when hand sanding, back the paper with some type of block, not the irregular tips of your fingers. For flat surfaces use a wooden block; for curved surfaces, use a rubber block or a flexible float. Wrap the paper around the block or attach it with disk adhesive.

If you do need to use your fingers when sanding on a surface—and occasionally you will—put on a good pair of cloth garden gloves, then fold a quarter sheet of paper into thirds. This ensures that the paper will not sand itself and that you get three fresh faces out of each piece.

Sanding Wood

Always sand with the grain; sanding across the grain introduces scratches that almost certainly will be accentuated by any finish you apply.

Always sand “through the grits.” Begin with a grit that is coarse enough to remove the worst defects, then sand the same surface with progressively finer grit paper until you get the smooth surface you want. In most cases you might start with an 80-grit paper and work up. If you are sanding a wood surface with planer marks, you might begin with a 60-grit paper, or even a 36.

When sanding teak, use 100-grit paper for the exterior woods. For interior work continue up to 120-grit. Other woods might need up to a 220-grit to create a good surface for varnishing.

Never skip a grit size when you are working through the grits. It doesn’t save time and often leaves sandpaper marks.

Sanding sponge

Sanding sponges are basically grit mounted on a thin sponge backing. They are easy on the fingers, amazingly long-lived and terrific for wet-sanding a bottom before applying a fresh coat of antifouling paint (assuming local regulations allow this). Wet sanding doesn’t create toxic dust, and because a sanding sponge follows the contour of the hull it can cut old paint quickly.
The basic rules for what sandpaper to use, and when, are not complicated. Dry sand with aluminum oxide. Wet sand with silicon carbide. Bottom sand with a sanding sponge.

If only everything else in life were so simple.