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Abrasives

used in woodworking projects

The generally hated process of sanding can be made easier by some machines and tools, described in the web page on sanding, but the performance of those tools (or the hand sanding done without the tools) is greatly impacted by the use of the proper abrasive.

Coating

Garnet is a traditional abrasive used to get a fine finish on wood - not for rough sanding. As I wrote this, I realized I hadn't seen any Garnet paper for sale recently, and as I raided my drawer of sandpaper, I couldn't find any. Not only has it become far less common than years ago, but maybe it has become obsolete. It is generally an orange color grit. I thought it's advantage was that it is friable - the grains break and give a new sharp edge as the paper wears. My recent research, checking facts for this page, suggests that it wears fast but is not friable.

Emery is next on the hardness scale, typically black, often cloth backed, and often used in metal working (and plumbing). Not for us woodworkers. In the past this was used in wet-or-dry applications.

Aluminum Oxide is next on the hardness scale. It is friable, thus self-sharpening, nominally making it last a long time. It is often used in it's natural brown color, available in 24 to 400 grit. This is one of the "workhorse" abrasives in today's technology. Sometimes it is coated with Zinc Stearate to reduce clogging.

Silicon Carbide is nominally the hardest abrasive, friable when used on ceramic, glass, and metal, but not friable on wood. Experts say it will dull and not last as long, but I like how long it lasts, perhaps because the grit doesn't break in routine use on wood. This is the second "main" abrasive in today's technology.

Having declared Silicon Carbide the hardest, the articles go on to talk about various zirconia compounds or ceramic being even harder, but only available in coarse grits. Several articles suggested that zirconia or ceramic were only available as fine as 120 grit, but catalogs are starting to offer this blue abrasive up to 150 or 180 grit, and as technology advances, I expect to see even finer grits emerge, and the premium price to fall.

CAMI vs. FEPA

There are two standards for abrasive grits...and two more ways of measuring. CAMI is the Coated Adhesive Manufacturer's Association (North America). FEPA is The Federation of European Producers of Abrasives. The scales are quite different, so the FEPA grits normally have a P with the number. Most abrasives in the United States are made by subsidiaries of European companies, so the P-grits from FEPA have become most common, and have been adopted by the International Standards Organization, ISO. The third measure is the actual size of the grits, measured in microns, so a small number is a finer grit, unlike CAMI and FEPA where a large number is a finer grit. The fourth measure is used for steel wool and equivalent, but the references I have found are consistently inconsistent (a definite maybe). Therefore use the fourth column only as a very general guide. Here are some comparative grits:
 
FEPA CAMI Avg Particle Diameter - microns Steel Wool or equivalent
P12   1815  
P20  1000  
P24  764  
  24 708  
P36   538  
P40 40 425  
P80   201  
  80 190 3M Green
P120   125 3M Brown
  120 115 3M Gray
P150   100 0
P180 180 82 00, 3M Maroon
P220 220 68 3M Gray
P320   46.2 Mirka Green
P360   Mirka very fine Mirka Maroon, 3M Maroon
  280 45 000 3M Blue
  320 36.0  
P400   35.0  
  400 23.0 000, 3M White*??
P800   21.8 3M Lt Gray;
    20 0000
P1200-1500   Mirka ultra fine Mirka Dark Gray, 3M White*
P2000   Mirka micro fine Mirka Gray
* An old reference called 3M White about P400; a newer reference calls it P1200, but another says the 3M Color for P1200 is Gray. I believe the common 3M white stuff is around P1200.

Keeping abrasive clean

In a closed coat abrasive, the grit covers most or all of the surface, and is used in metals and rubbing finishes. An open coat typically has 40% to 60% grit coverage, to reduce clogging the grit with the sawdust (or in the case of metal, swarf). I presume it works, so fine, but I feel like I am getting less for my money!

Stearate (of some sort, such as Zinc Stearate) is a coating that is put on some sandpapers (and in some sanding sealers) so that when the material is sanded, it powders much like talcum powder, and does not clog the sandpaper. For years, it was nicknamed "soap" since it made sanding seem slippery and easier. "Normal" stearate interferes with water-based furniture finishes, but some vendors, including Mirka and apparently Klingspor and possibly others, use a modified stearate that does not bother water-based finishes. Without the stearate, the finishes don't sand as easily, and the paper clogs more. The auto industry uses finishes with solvents that are not bothered by silicone or stearate. Furniture finishes generally don't tolerate silicone or the original stearates, so be careful if using automotive abrasives, rubbing compounds, or waxes.

Cleaning sticks - some people use an eraser-like cleaning stick to clean their sanding belts and discs. My experience is that it makes the belts look pretty but doesn't remove the tiny specks of resin (or not-so-tiny streaks) that adhere to the belt and destroy it's effectiveness, leading to burning. I have cleaned drum sander belts with a power washer, on my driveway, and let them dry in the sun (significantly extending their life), but I have discarded my "eraser." I hear there is a procedure sometimes used in factories to shoot powdered dry ice (from a CO2 fire extinguisher?) on a wide belt while it is running, which makes the resin brittle so it breaks off the belt.

Disks with Dust Collection

Five inch discs are quite common. Six hole is getting rare, 8 holes is common, and Norton has a disc with funny shaped larger holes that fit either a 6 or 8 hole sander (but the large holes leave the hook and loop pads exposed over a relatively large area).

Festool 5 inch disks have their usual 9 hole pattern with 8 holes near the edge and a center hole that allows air to enter, presumably allowing the other 8 holes to suck more efficiently.

6 inch: If there is a standard 6 inch disc, it is probably this with 6 holes, fairly far in from the edge. Some list this pattern for Porter Cable and Bosch. If I see 6 inch paper locally, such as at the Home Depot, it is this hole pattern.

Eight hole 6 inch: I have not seen this in use, but I have seen it listed for Fein 6 inch sanders. Note the apparent similarity with 8 of the holes in the new 6 inch Festool discs below.

Festool 6 inch: Like their 5 inch disc, Festool has their unique 9 hole pattern with 8 holes near the edge and a center hole that allows air to enter, presumably allowing the outer 8 holes to suck more efficiently. This 9 hole pattern is becoming obsolete - see the 17 hole pattern below.

Festool has changed the design of their 6 inch (150 mm) sandpaper and sanding pads, from 9 to 17 holes - the 17 hole paper will work on a 9 hole pad, and if you buy a new 17 hole pad you will be able to use up the older 9 hole paper with the new pad. The sanding pads have the center hole and two rows of 8 holes. The inner row of holes is roughly the diameter of the common 6 hole pattern, but cannot be used on other 6 inch sanders because the common 6 hole standard will not align with Festool's 8 holes.

Dust Collection

Dust can be harmful to your health and the work in your shop, and the fine dust produced by sanding is the worst kind. See the special web page on dust collection. Shop air filters are not a good solution - they do only a fair job of collecting the larger particles (which are less of a health problem), and stir up the almost invisible fine particles that are worst for your health.

Downdraft tables are work tables with holes in the surface connected to a high volume dust collector that moves the air from the work area, down through the table, and into the main dust collector (which hopefully is effective at filtering both large and small dust particles. This is a good solution if you are doing a lot of hand sanding, or are in a factory where you can afford a dedicated sanding area, but I have rarely seen a small shop dedicate the required space.

Dust Collectors are virtually required on larger sanding machines. Years ago I started collecting dust to use as filler, in a pint or smaller jar for each wood species. Now I can fill a jar in one or two passes of a board on my large sander - the amount of dust produced by a big sander is absolutely amazing, and if it isn't collected as it is produced, it will destroy the abrasive (and slowly kill the operator).

Collection Bags on hand sanders are fairly good - vanes on the sander motor create an airflow to pull the air through the abrasive and into the collection bag. I would certainly use a collection bag rather than nothing.

Shop vacuum - the high quality low noise automatic type - is one of the better investments I have made. I have removed the collection bag from most of my sanders, and have mounted a connector on each of my sanders that allow me to plug in the vacuum to the sander I am using. A shop vacuum has much more suction than a few vanes on the sander motor, and dramatically improves the dust collection. I use an extension cord from the vacuum to the sanding area, so multiple sanders can be plugged in, and using any of them starts the vacuum. To switch sanders (often to choose a different grit), I just move the hose and go! I chose a small vacuum to take minimum space in the shop (the smallest automatic Fein at the time, but Festool and others are good), and use my big dust collector to empty it when necessary. Of course, my clothes are still dusty when I leave the shop, even with the vacuum on the sanders - it isn't perfect.

Attaching the sandpaper to the sander

PSA, or Pressure Sensitive Adhesive, is widely used commercially, where it is rarely necessary to change to a different grit before the abrasive is worn out. The flat surface and adhesive does not wear out like the Hook and Loop and StickFix options.

Hook and Loop (think velcro) is widely used with small hooks on the sanding pad, and a soft fabric (like flannel or thin felt) on the back of the abrasive. The hooks on the pad are subject to wear, so the sander pad (not cheap) may need to be replaced occasionally.

StickFix is Festool's extra long hook on their sanding pad, that presumably holds the abrasive tighter and lasts longer. However, the hooks are long enough to extend through Abranet sanding screens, and are rapidly worn off. If you use Abranet on a Festool sander, buy an interface pad that hooks normally on the Festool sander, and has shorter hooks for the Abranet abrasive.

Drum sander note: On my Performax/Jet drum sander, a cloth abrasive belt was stretched around a metal drum. Combined with a thin sandpaper-like feed belt, the thickness of the work piece could be very precisely controlled (unless the belt stretched, slipped, and overlapped at some point). On the large drum sander that replaced the Performax, the cloth abrasive belt is held on the drum with hook and loop technology. Combined with a thicker rubber feed belt, I can make three passes through the sander, still removing material, without adjusting the height of the drum. Clearly the softness of the feed belt and the thicker, softer hook and loops reduces the precision of the thickness of the work piece.

Backing

If we want "sandpaper" we need a paper or cloth or other flexible material to which the grit can be attached. In simple terms, "A" is the thinnest or lightest backing, and "Z" (or at least "Y") is the heaviest. To help calibrate, I made this list from several paper and on-line catalogs - I am sure other lists use other letters, but these are the ones I found:

If you want to use an abrasive without a backing, it is called a "rubbing compound." The most readily available are used in the auto industry, but they often include silicone compounds which interfere with many finishes, especially water based finishes. If you are using a rubbing compound it is probably because you want a finish finer than available with sandpaper. Such a fine finish requires a VERY smooth and flat surface - be sure to fill the wood grain and any flaws in the surface before starting to apply the finish.

Menzerna of Germany has a line of compounds for woodworkers, used by Steinway and Baldwin for their piano finishes as well as guitar companies like Fender, Martin and Taylor. I started using these with the recommendation of Jeff Jewitt and others, and have been very pleased with the results. Some rubbing compounds reportedly contain chemicals that impact new-tech finishes (leaving a white haze); Menzerna does not have that problem. There are three Menzerna grades, used in sequence

The Menzerna compound is often applied with a felt pad (on a ROS), or with a Surbuf pad (dedicate a pad to each compound to avoid contaminating your work.) Surbuf sponge pads are sold in pairs, fabric on one side that sticks to a hook-and-loop sander, and with fibers that stick straight out of the other side. They work well with rubbing compounds and water.

Steel Wool and equivalent

For years the "standard" of fine finishing, including rubbing out a film finish, was steel wool - often the very fine 0000 grade. However, microscopic fibers of steel can be embedded in the finish, and when repaired (or the finishing continues) with water-based finishes, the steel fibers can rust. Therefore the traditional steel wool has largely been replaced by non-woven synthetic abrasives from 3M and Mirka.

The 3M pads appear to be sharp plastic without a separate abrasive, and are coded by color - green is very coarse (used to clean the burned frying pan) - about P100 grit. The next is Maroon, about P180 grit. Gray is next at about P220 grit, and finally white at about P600 or P1000 grit.

Mirlon pads from Mirka have an abrasive attached to the unwoven pad, which also has a color code. Some vendors only sell a few of their four grades - Green is called "general purpose" and is about P320 aluminum oxide, Red/Maroon is called "very fine" - about P360 aluminum oxide, Dark Gray is called "ultra fine" - about P1500 grit silicon carbide, and Gray - "microfine" is about P2000 silicon carbide. Note that these are much finer grits than most rubbing pads, and I have found that they last a long time.

Major vendors

All of the major abrasive companies have North American or US subsidiaries, but many are based in Europe.

Sources for unusual abrasives

The best (cheapest) source I have found for Mirka Abralon (the sponge backed fine grit abrasives used for wet or dry rub-out) is Mirka Online, which appears to be a division of Beaver Tools. Abralon is only available in 3 and 6 inch discs, but the 6 inch discs can be used on a 5 inch sander. The 6 inch discs come in boxes of 20, but you can order 5 at a time from this web site.

If you decide to mail order Mirka Abranet, it is also available from Mirka Online. Search for 9a-232 for the 5 inch discs of for 9a-241 for the 6 inch discs in packages of 10 or 50. However I was contacted by another company, 2Sand.com (that I have not yet ordered from) whose gimmick is to include a "free" interface pad (good idea) with each box of abranet.

Mirlon synthetic steel wool is available in abrasive sheets from Woodcraft, and is available as disks for a random orbital sander from Mirka-Online.