It’s easy to think that surfaces prepared on jointers and planers are ready for finishing, particularly when working with wood that has a straight grain and a fine, even texture. Surfaces may look and feel quite smooth, but closer inspection almost always reveals the presence of minute grooves, or ripple marks, on the wood’s surface. There are also likely to be areas of tearout. While the marks may be very small, they need to be removed in order to ensure a first-class finish. Don’t think that machines equipped with segmented (aka helical or spiral) cutterheads do any better – they also produce ripple marks that you will want to remove. The principle here is that machinery (including routers and shapers) are used to dimension your stock, not to prepare surfaces for finishing.
That leaves three methods for surface preparation: sanding, scraping, and hand planing. Sanding is, by far, the most commonly used method, primarily because it’s the easiest to learn and the quickest to achieve great results.
Sanding isn’t difficult, but it is time consuming. To get the very best finish you need to use the right sanding abrasive in the right sequence, the right way – regardless of whether you’re sanding by hand or with power tools. The aim in sanding is to use sandpaper with increasingly finer grits of material to remove surface blemishes and achieve as close to a scratch-free surface as required before applying your chosen finish. In this article we’ll begin by looking at sandpaper. In future articles we’ll cover tools and techniques for hand and power sanding.
When sanding oil-based finishes you can use a special lubricated paper to help protect against clogging. The residue can contaminate waterbased finishes though.
The four basic types of grit
Aluminum-oxide (AlO) grit
is the most widely used abrasive material. The grit comes in various grades with the better quality being more friable. As the grits break down they retain their sharpness so the sandpaper cuts quicker, doesn’t load-up as quickly, and lasts longer than most other sandpapers. In use, the grit becomes smaller in size, producing a somewhat finer scratch pattern. Grit sizes range from about 40 to 800, and it’s available in all the common formats.
Silicon carbide (SC) grit
, also referred to as carborundum, is harder than AIO, but not as durable. Under pressure it tends to wear down quickly. However, it cuts more rapidly and produces a uniform scratch pattern. It also has a waterproof backing. SC is typically used in finer grits for smoothing a finish between coats and for rubbing out film finishes, where lighter sanding pressure is applied. It’s generally available in grits 220 and above, in sheet format.
is the granddaddy of sandpaper grits. It’s a natural grit with hard, sharp edges that aren’t friable, but crumble in use, which tends to burnish the wood surface. It wears more quickly and has a greater tendency to clog up than AIO, and is bonded to a stiff paper backing that breaks easily when folded. However, it does produce a smooth finish, and is about 50% less expensive than AIO, which still makes it a popular choice among woodworkers. Garnet is typically available in sheet form up to 220-grit.
, in the form of alumina zirconia and ceramic aluminum oxide, are very tough, sharp, and long-wearing. The grit is friable, but much less so under hand pressure. More expensive than other grits, they’re generally available in belt and disc formats from 36 to 120-grit, and used primarily for power sanding.
Some manufacturers, like Festool and Bosch, have created their own line of sandpaper that works nicely with their sanders.
All sandpaper consists of natural or synthetic abrasive material embedded on a substrate, the backing, which is typically paper, cloth, or film. The abrasive bonded to the substrate is referred to as the grit (or grain). As you rub sandpaper over a wood surface the grit produces a scratch pattern. The finer the grit that is bonded to the sandpaper, the smaller the scratch pattern.
As you sand, the grit breaks down. Some grit fractures into smaller pieces, exposing fresh cutting edges to scratch the wood – a process called friability. Most grits are not friable – they crumble into smaller blunt pieces, so cut more slowly and wear away more quickly.
The backing for paper runs from A-weight (the lightest) to F-weight (the heaviest). The two most common cloth backings are J-weight (light) and X-weight (heavy). A strong backing material won’t easily rip or crack as it’s bent. In general, the stiffer the backing the less the grit will flex under load, resulting in more aggressive sanding with a more prominent scratch pattern. Thinner backings are typically more flexible, allowing the grit to compensate for the inconsistencies in the surface of the wood, giving a finer scratch pattern. Wet/dry sandpaper has a paper backing impregnated with latex and can be used to wet sand between coats of finish or to rub out the final finish.
Single sheets, packs, discs, rolls, sponges, hook & loop, PSA backed, garnet, aluminum oxide, or silicon carbide, your local Home Hardware store or Home Hardware Building Center has the abrasives you need at prices you can afford. HomeHardware.ca
Sandpaper can be close-coated – the backing is completely covered with grit, which tends to clog up the sandpaper with dust – or open-coated – gaps are left between the grits, which allows sawdust to gather so the sandpaper doesn’t load up as quickly. In addition, some sandpapers are classified as no-load or lubricated – they’ve been treated with a stearate coating, a soapy substance that helps prevent clogging, reduces heat build-up, and makes the sandpaper last longer. Of course, the coating will eventually wear off. That coating can also contaminate some finishes, so it’s good practice to wipe down wood surfaces after sanding with stearate sandpaper – when applying a waterborne finish it’s best to not even use stearate sandpaper on the piece.
Woodworkers use sandpaper in different formats: sheets, discs, belts, rolls, and pads. Discs are available in two styles for attaching to the backing pad on a power sander. PSA (Pressure Sensitive Adhesive) discs use an adhesive while hook & loop discs use a Velcro system. Hook & loop discs are far quicker and easier to use. Sponges, made with open-coat aluminumoxide grit, come in various shapes, and are ideal for sanding curved work, profiles, and intricate shapes.
A few recommendations
There are quite a few brands of sandpaper on the market. Most brands can be purchased in singles or small packs, either sheets or discs, which makes it convenient and economical to try the various products so you can hone in on the ones that do the job for you. While some brands, such as 3M, Mirka, and SIA, manufacture their own sandpaper, others offer generic products, often manufactured offshore. As well, both Festool and Bosch market their own line of sandpaper.
The products I particularly like for general sanding by hand and power tool are Norton’s ProSand and Mirka Gold, both available in sheets and discs up to about 800-grit (sheets) and 400-grit (discs). These stearated AIO sandpapers have strong, flexible backings, don’t wear out too quickly, and don’t overly clog-up with dust. The ProSand discs are covered with tiny holes that really help with dust extraction. Another good choice is 3M’s No-Load Sandpaper, with an open-coat, stated AIO coating in sheets up to 800-grit.
For sanding contoured shapes, narrow grooves, detail work and the like, I use 3M Ultra-Flexible Sanding Sheets. These have AIO grit bonded to a super thin film, making them extremely pliable. They’re available in sheets from 100 to 320-grit.
Another option that I’m using a lot more frequently is the Mirka Abranet discs. They use an AIO grit on a strong, flexible polyamide nylon fabric mesh backing. They cut very fast, are long lasting, and the mesh backing provides superior dust extraction. The discs are available in grit sizes up to 1000-grit.
UAMA or FEPA?
Sandpaper is classified primarily by the type of grit and the size of the grit. There are two common standards for classifying grit sizes – UAMA (United Abrasive Manufacturers Association) and FEPA (Federation of European Producers of Abrasives). UAMA graded sandpaper is printed with the grit size number on the backing, FEPA with a ‘P’ preceding the grit size (on the backing and sometimes just on the packaging). Sandpaper made in Europe confirms to the FEPA standard. Up to about 220-grit the two classification systems are very similar, otherwise they are not at all comparable. For example, FEPA P1500-grit is approximately equivalent to UAMA 800-grit.
The sequence of grits used will depend on such factors as the prominence of milling marks left after jointing and thickness planing your stock, the grain pattern of the wood being used, and the type of finish that will be applied. In general though, a good place to start is with P80-grit or P100-grit to remove machine milling marks, followed by P150-grit if a film finish is to be applied, and up to P220 for an oil finish. Up to P320 on end grain and blotch-prone woods.