Sandpaper, grinding discs, and cut-off wheels are items we all have and use regularly out in the shop, but they’re items we all take for granted and use without much thought. Recently, while reloading my grinder with a new disc, I happened to take a closer look at the one I was loading up and wondered what the story was behind it (perhaps being a new fan of that cable TV show “How it’s Made” has piqued my curiosity). Anyway, I went ahead and finished what I was doing, but also decided to do a little investigating regarding abrasives. The following is what I learned and thought was interesting enough to share – and I hope you agree.

Abrasives like the sandpaper or grinding discs operate pretty much like cutting tools. The rough particles on sandpaper and discs are a coating of sharp-edged materials that cut similar to the way a file does. But sandpaper is sandpaper, right? Well, not exactly. There are actually two different types (or grades as they’re called in the industry) of sandpaper on the market – “commercial” and “industrial.” The commercial grade is the stuff commonly available at hardware stores and places like Home Depot, etc. The industrial grade is usually available only through industrial supply stores and/or autobody supply jobbers. The industrial-grade abrasives found in the latter outlets are made from higher-quality materials and are designed to be used in production or commercial situations. In other words, this type of abrasive (commercial) is the heavy-duty version – and the type we should be using.

OK then, let’s start with my first question, namely, what is grit? When talking about abrasives, “grit” is a reference to the number of abrasive particles per inch of sandpaper. The lower the grit designation or number, the rougher the sandpaper – and conversely, the higher the grit number, the smoother the sandpaper. It makes sense if you realize that fine sandpaper like 1,200-grit, for example, would require 1,200 tiny particles of abrasive to fit within one square inch of surface – they’d be pretty darn small. At the other end of the spectrum might be 36-grit, which would only have 36 big (relatively speaking) chunks of abrasive per square inch – pretty darn rough compared to 1,200-grit.

OK then, knowing this, what should I use for what?

The grit you use depends on the job you need it to do. For example, 16-24-grit sizes are generally reserved for hard-backed grinding discs and are usually used for stripping heavy rust or multiple coats of paint off heavy or thick material (not sheetmetal or thin stuff cuz they’re so aggressive they’d cut right through it if you aren’t extremely careful).

With 36-60-grits, these are considered coarse, but are usually the starting point for sheets of sandpaper (though these grits are also available in hard-backed discs, as well). Still considered pretty darn aggressive, these grits can still be used for heavy sanding and stripping of heavy material, but can be used on thin material, too. Just remember to be careful if these grits are used in conjunction with a sanding machine of any sort versus doing it by hand.

80-120-grits are considered medium, and used primarily for smoothing a surface and/or removing smaller imperfections and marks. Grits in the 150-180 range are labeled as fine and useful for a final sanding pass before a primer coat, for example. 220-240-grits are designated as very fine and are good for sanding between coats of primer or sealer. 280-320-grits are labeled extra fine and are good for removing dust spots or fine scratches between finish coats. 360-600-grits are considered super fine, and are used for sanding a surface to remove some luster or surface blemishes and finer scratches. 800-2,000-grits are designated ultra fine and are usually reserved for final color-sanding on paint or for final finishing of metal that’ll remain uncoated. OK, so now that we’ve covered the basics of grit, we’ll look at the difference between commercial- and industrial-grade abrasives.

There are three main components to sandpaper: the abrasive grit, the backing material, and the bonding agents that attach the grit to the backing (be it discs or paper). Industrial-grade sandpaper uses higher-quality (heavy-duty) components as well as tighter manufacturing tolerances. Industrial-grade sandpaper also features harder and sharper grit that is less likely to break down or wear out, better bonding agents that hold the abrasive to the backing material, and heavier-duty backing that is less likely to tear or wear through.

Choose your weapon

Metalworkers and body men use a sanding procedure sometimes called “going through the grits.” Not to be confused with a good old southern breakfast, it really refers to the process of sanding where one uses progressively finer pieces of sandpaper to get a smooth finish. By going through the grits, each progressive sanding step removes the scratches left over from the previous step. Skipping grits to save time is not necessarily a good idea as you’ll usually end up sanding longer just to remove the scratches left by the previous grit. You’ll always want to start with a low, coarser grit, and then work your way up to a finer grit.

There are also different types of sandpaper, specifically “open coat” and “closed coat.” Open-coat sandpaper has gaps and open spaces between the grits that help prevent clogging by giving the removed material space so it can separate or fall away from the paper or disc. Closed-coat is better for sanding metal finishes as it’s more aggressive, but clogs a bit easier on softer materials like primer or some body fillers.

So, though not a complete course on abrasives, hopefully this’ll give ya a bit of a background on a common tool we all use without much thought, and a basic guide to sandpaper and sanding disc choices.

SOURCE
Summit Racing Equipment
PO Box 909
Akron
OH  44398
800-230-3030
330-630-0240
www.summitracing.com
Eastwood Company
263 Shoemaker Road
Pottstown
PA  19464
800-343-9353
http://www.eastwood.com
Wet Wedge
Irvine
CA
949-295-5824
www.wetwedge.com
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