Choosing and Using Sandpaper
Here's the secret to getting good results from woodworking's humble sandpaper.
The flawless gleam of an oak floor, the smoothness of a painted wall or ceiling, the high shine of a varnished tabletop — all are signs of a job done well. And all are made possible by the patient, methodical application of sandpaper.
It's a task many of us wish we could avoid or cut short. Even This Old House master carpenter Norm Abram.
"Pushing sandpaper often seems like an endless job," he says. But without proper sanding, any imperfections will be magnified when the final finish coat is applied.
The secret to getting good results with less tedium is to choose the right paper for the job. Sandpaper works by scratching away defects with thousands of tiny abrasive particles. The bigger the particle, or grit, the bigger the scratches. Hence the iron rule of sanding: Start with a grit coarse enough to quickly remove surface imperfections and follow with incrementally finer grits. Each successive grit erases the scratches of the coarser one before, until the scratches themselves become undetectable to the eye and the touch. To prepare bare wood for paint, for instance, Norm starts with 80-grit paper, followed by 100, 120, 150, and 180, and finishes up with 220. (With most sandpapers, the coarser the grit, the smaller the number.) The temptation is to skip a grit in the sequence. "People get impatient," Norm says. "They think a surface feels okay, but after the finish goes on, every flaw and scratch pops out."
For every sanding chore, there's a paper to suit:
•Sanding screen (silicon carbide) for clog-free smoothing of joint compound
•Sanding belt (ceramic aluminum oxide) for fast removal of stock
•Flexible sanding sponge (aluminum oxide) for contoured surfaces
•Sanding disc (aluminum oxide) for smoothing flat surfaces
• Cloth-backed wet-or-dry sheet (aluminum oxide) for smoothing clear finishes; uses water as a lubricating film
•Paper-backed sheet (garnet) for all-around wood-sanding tasks
Early sandpapers were made of natural minerals like pumice, flint, garnet, and emery fixed to a paper backing with glue or varnish. Nowadays, most sanding abrasives are synthetic materials — aluminum oxide, silicon carbide, and high-tech ceramic — that stay sharp longer and can be produced in microscopic sizes. Deposited onto backings of paper, cloth, or plastic, they are held in place with tenacious plastic resins that can withstand the heat and speed of machine sanding.
With better technology has come a host of specialized abrasive products: sanding belts and discs engineered to withstand the rigors of rapid stock removal without clogging; sanding blocks that flex to follow irregular contours; plastic-film-backed "papers" so fine they can gently polish away defects without harming furniture's finish. Norm takes full advantage of these improved papers, using his orbital, random-orbit, and belt sanders to shorten the time he spends at the task. But he always finishes up by hand, with a sheet of sandpaper wrapped around a sanding block. "A power sander can still leave little swirl marks," he says. "Nothing beats a final hand sanding for making sure a surface is smooth."
The amount of abrasive on a piece of sandpaper affects the way it performs. "Open coat" papers have more space between each abrasive particle and so don't clog as quickly with dust. (Coarse grits are typically open coat.) In grits finer than 150, where clogging is less of a problem, most sandpapers are "closed coat," covered completely with abrasive. Hardwoods and metals can be sanded with closed-coat papers; open coats are best for soft woods like pine.
In addition, some papers are treated with zinc stearate, a soapy substance that prevents clogging. Don't use these so-called "nonloading" papers with water-based finishes, however; stearates can prevent the finish from adhering properly.
*To remove mill marks and deep defects, start with 80 grit
**For paint in poor condition, 100, followed by primer
All About Abrasives
Once, sandpaper came in two colors: black or tan. In recent years, manufacturers have begun to tint their abrasives with dyes of lilac, teal, or burnt umber to make them stand out from the competition. Now only the natural abrasives — garnet and emery — can be counted on to always display their true colors. Following is a guide to the six most common types.
Suitable for sanding bare wood. Dulls quickly compared with man-made abrasives.
Excellent for sanding or polishing metal; too soft for other uses. Comes on flexible cloth backing.
Extremely hard; sharp enough to cut glass, but wears quickly. Best for smoothing joint compound and removing dust nibs between coats of finish. Extra-fine grits are used for wet sanding the final finish coat.
The jack-of-all-trades abrasive; great for hand or power sanding on wood, paint, or metal. Not as sharp as silicon carbide, but lasts longer.
Tougher, more durable, and more expensive than other abrasives; often bonded chemically with aluminum oxide. Used primarily on belts and discs for power sanding.
An alloy of aluminum oxide and zirconium oxide. Sharp, hard, and durable, it cuts faster and lasts longer than aluminum oxide, but isn't as long-lived as ceramic. Found mostly on belts and discs for machine sanding.
Knowing what grit to start with and when to stop is the key to a perfect sanding job. Starting with too coarse a grit leaves deep scratches that are tedious to sand out. But starting with too fine a grit eats up time and paper, as does finishing up with a finer paper than is necessary for the job. Here are some guidelines:
Reading a Sheet
Time was you could read the back of a sheet of sandpaper like a book. Printed there were the grit size and type, the weight of the paper, the kind of glue used, and whether the sheet was open or closed coat. Now, about the only thing you'll consistently find, other than the manufacturer's logo, is the grit size. But even that bit of information can be confusing. Sandpapers in the U.S. have traditionally been graded according to the Standard Scale, in which the the numbers grow larger as the grits get finer. A "P" in front of the grit number designates a European system, which closely corresponds to Standard grades up to 240 grit, but diverges as the grits get finer. (A P800 is the equivalent of a Standard 400.)
A grit number followed by the Greek letter µ (pronounced "mew") indicates micron grading, in which the numbers get smaller as the grit gets finer. (A 60µ is about the same as a Standard 240.) These pricey "papers" — their backing is actually plastic film — are prized by auto-body refinishers and furniture makers.
Going Through the Grits
Sanding is a step-by-step process of scratching a surface with progressively finer abrasives. Each successively finer grit abrades away the marks left by the previous, coarser one, until the surface looks and feels smooth.
Where to Find It
St. Paul, MN
Ceramic, Aluminum Oxide, and Alumina Zirconia