True Grits: What you need to know about sandpaper for a faster, finer finishComments (0)
This article is from Issue 21 of Woodcraft Magazine.
What you need to know about sandpaper for a faster, finer finish
By Joe Hurst-Wajszczuk
“Running through grits” not only means going from coarse to fine, but also matching the abrasive to the job. Before you stockpile any more sandpaper, make sure what you do buy (and apply) is the right stuff.
Despite its importance, sanding may be the most under-appreciated step in the project-building process. Few of us enjoy it, even fewer enjoy talking about it, but no one can deny the contribution it makes to the look (and feel) of a finished project. In most shops, sandpaper does a lot more than just raise sawdust. The right abrasives—in the appropriate grits—can help rub out brush marks and dust nibs from finished surfaces, scrub rust from machines, even sharpen planes and chisels. Add up the cost of a career’s worth of disks, belts, sheets, and rolls and you might find the price tag approaching the cost of a cabinet saw. All good reasons to give sandpaper a closer look. This primer can help you finish faster and with better results.
All about abrasives
Although the teeth are significantly smaller, sandpaper is a cutting tool not unlike saw blades and router bits. With each back-and-forth pass of the sanding block or disk rotation, the abrasive grains work like miniature cutters, shaving off hundreds of chips (called swarf). Understanding the cutting qualities of each abrasive type can help you find the right sandpaper for the job.
Don’t let all those colors in the sandpaper section or catalogs throw you. Abrasives boil down into four main categories. From there, manufacturers offer different combinations of backings and bonders to help the abrasive cut longer without clogging or falling off.
Sand smarter, not harder
Like the food at an “All-You-Can-Eat, $3.99” buffet, more sanding isn’t necessarily a good thing. You may be able to stop sanding sooner and achieve the same results.
Realize that sanding’s primary purpose is erasing mill marks and other surface blemishes. For a film-forming finish, 150 or 180 grit is usually fine enough. For oil, 220 grit will often be fine enough, but for blotch-prone woods such as cherry, you may want to step up to 320 or 400 grit.
Cost: $ (Least Expensive)
Old-school woodworkers claim that the naturally occurring mineral produces a “softer” scratch pattern. Their claims may have a few grains of truth. The older CAMI (Coated Abrasives Manufacturer’s Institute) grit grading allows a wider particle-size spread than the FEPA (Federations of European Producers of Abrasives) grading system. (See “Making Sense of Sandpaper Scales,” page 59.) A few rogue grains can create scratches even as you sand others out. In this case, garnet’s relative softness is an advantage. Unlike harder abrasives, the larger grains are more likely to break out before creating coarser scratches. This attribute can help create a more consistent scratch pattern. Garnet crystals are friable, meaning that they fracture in use, exposing additional small, sharp edges. Garnet wears quickly, making it less suitable for heavy sanding or for metal work. In addition, water-soluble hide glue is used to attach the abrasive to the paper. This makes the paper unsuitable for wet-sanding. In humid conditions, the grit can flake off the paper.
2 Aluminum Oxide
Cost: $-$$ (Price varies with quality)
The do-it-all abrasive, Al-O is suitable for wood, metal, cured finishes, even stripping off old paint. However, within this category you’ll find the widest range of quality and performance. The abrasive used on the least-expensive paper does not fracture easily; instead the grit dulls or simply flakes off. This can lead to an inconsistent scratch pattern.
Better-quality sandpapers employ a more friable abrasive for a longer cutting surface. Higher-quality papers also use better bonding resins to hold the grit onto the paper. This is especially important when machine-sanding. Manufacturers use different ranking systems to differentiate their own products, but to compare brands, you’ll need to do your own sanding tests. You can sometimes tell the good from the great abrasives by checking the price tag (better backings and abrasives cost more), but realize that you can spend less for the good stuff if you buy in bulk.
Check your paper before finish-sanding. Some aluminum-oxide sandpapers are designed for wet-sanding; others contain special clog-busting coatings called stearates and are designed to be used dry.
3 Silicon Carbide
Silicone carbide is good for fine-sanding metals, plastics and rubbing-out finishes. Glass-sharp grains cut quickly but are less suitable for heavy-duty sanding or metal grinding as when flattening chisel backs or plane blades. In these cases, the grit will wear quickly or shear off the paper. For serious sanding, look for aluminum oxide or a synthetic abrasive.
Most silicon-carbide sandpapers are matched with waterproof bonders and backings so that they can be used with a sanding lubricant. A few drops of water or oil can help remove the sanding swarf that might otherwise keep the paper from cutting, or caking up and creating unwanted scratches.
Cost: $$ - $$$ (Most Expensive)
Industrial-grade abrasives such as aluminia zirconia AND CERAMIC ALUMINUM are the most expensive, but make sense in heavy-duty applications. These abrasives are harder and hold up to heat better than ordinary aluminum oxide, an advantage when doing serious power sanding or major metal work, such as cleaning up the sole on a rust-pitted jointer plane. Many also are equipped with stearate coatings to prevent clogging when sanding resinous woods or finishes.
Although these high-tech abrasives work well for power sanding, the extra investment may not always offer a worthwhile return. Low-speed hand-sanding doesn’t make use of the abrasive’s heat- or abrasion-resistant qualities like a disc or belt. When sanding turnings or curved surfaces, or when wet-sanding a finish, you may still want to use a less-expensive sandpaper and toss the worn-out scraps when you’re done.
Special Sandpaper 1: Micro-Mesh
This abrasive isn’t new, but Micro-Mesh’s special sanding properties and unique grit grading scale deserve extra explanation.
With regular sandpaper, a flat back keeps the abrasive on the same level plane for a more consistent cut. Micro-Mesh solves the scratch problem by going in a completely different direction. The cushioned backing works like a pressure-safety valve, allowing the grit to sink in so that the cutting tips contact the sanding surface at the same level. This grit-backer combination results in a more refined scratch pattern. (Although it’s great for polishing, the cushioned backing makes the paper less aggressive. For removing deep scratches, you’ll want to switch over to regular sandpaper.)
Additionally, the flexible adhesive used to bond the grit to the backing allows the crystals to rotate instead of shearing off prematurely. Like wet-or-dry paper, Micro-Mesh can be used for wet-sanding, or you simply flush the swarf from the sandpaper at the sink.
Special Sandpaper 2: Abranet
Mirka’s Abranet employs a mesh backing, similar to drywall sanding screen, which works as the ultimate open coat. As soon as material is cut from the surface, the dust gets sucked through one of the thousands of holes in the mesh and straight into your shop vac. By preventing crud from caking up on the cutting surface, the mesh discs can cut faster and last longer than regular sandpaper. The mesh backing means superior dust extraction to help make a cleaner shop.
Abranet is available in sheets, but to make best use of through-venting mesh, outfit your random orbit sander with the 5" or 6" discs. The discs can stick onto hook-and-loop bases, but you may want to buy a new multi-holed backing pad to take full advantage of the mesh.
The interface pads are also optional, but if you’ve ever worn out the hooks on a sanding pad and been left with a disc-chucking sander, you’ll appreciate the investment. Used between the pad and disc, the interface provides a sacrificial set of hooks, protecting your pricier pad.
The backing material (cloth, paper, or plastic) that the abrasive is adhered to is almost as important as the grit
itself. The backing material performs a balancing act between flexibility, stiffness and durability—a combination that works for one type of grit or machine may not perform as well with another. In most cases, you’ll find that the sandpaper companies have already done the selection process for you, and have matched the grit to the backing that best suits your needs. However, a little information helps you fine-tune your sandpaper stockpile.
Cloth offers the most flexibility and durability, making it the best choice for belt and disc sanders. Heavy X-weight and lighter J-weight cloth backings are stiff and strong, but not as flat as paper or plastic, making them less suitable when polishing with finer grits.
Although not as long-lived as cloth, paper’s versatility makes it suitable for the widest range of grits. Paper comes in five different weights: A, C, D, E, and F. Stiffest E-weight papers deflect the least and offer the fastest cut. Lightest A-weight papers are reserved for the finest grits. Mid-grade C-weight is paper is commonly used for random-orbit sanders.
Film backings are more expensive than paper, so you won’t find them used in many instances. However, the plastic backing is flatter and stiffer than paper, making it especially well-suited for the finest grits. You’ll likely to find it used with abrasives designed for sharpening and super-fine finish sanding.
Additionally, manufacturers offer abrasives in cords, pads, and sponges in a variety of different densities and thicknesses. These choices are less about controlling the cutting-depth of the abrasive, and more about getting the grit in contact with the work that needs sanding, nooks and crannies included.
Making Sense of the Sandpaper Scales
The number on the back of your sandpaper doesn’t always provide the apples-to-apples comparison. You might think you're “running through the grits,” but without understanding the grit-grading systems, you might accidentally use a coarser paper.
Years ago, most of the sandpaper sold in the United States was graded on the CAMI scale. This standard was so commonplace that sandpapers simply listed the grit number on back. In Europe, the FEPA had its own metric grit-grading system. FEPA-graded paper carried a P prefix in front of the grit number.
Today that’s not always the case. Now that most manufacturers have switched to the FEPA-grading system, the letter P isn’t always stamped on the paper. Although most abrasives now sold use the FEPA grading system, you may still run into problems if you’re using up stockpiled sandpaper.
Although similar, the cross-continental grit-grading systems are different enough to create sanding problems, especially when finish sanding. As shown in the scale at right, the grit-ratings line up to 220. From there, the FEPA-grade gets increasingly coarser than the similarly-numbered CAMI paper. For example, the 400-grit CAMI paper is finer than 600-grit FEPA.
An easy and affordable way to add new life to old belts and discs is with an abrasive cleaning stick (Woodcraft #04W11, $7.99). Simply press the block against the moving belt or disc to unstick caked-on wood and resin. After a few seconds’ worth of contact, your old belt or disc will work almost as good as new.
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