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 of a thought. Well, 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 we all work with out in our shops operate pretty much like cutting tools. The rough particles on sandpaper and discs are a coating of sharp edged materials that cut much the same way a file does. But, sandpaper is sandpaper, Right? Well not exactly. There's 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 quality parts companies like Summit Racing Equipment or the auto body supply jobbers who supply professional auto body shops. 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.

Okay 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 - that'd only have 36 big (relatively speaking) chunks of abrasive per square inch - pretty darn rough compared to 1,200-grit.

Okay 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 of heavy or thick material (not sheet metal or thin stuff cuz they're so aggressive they'd cut right through it if you aren't extremely careful).

36-60-grits are considered as course, 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 that one still has to be careful if these grits are used in conjunction with a sanding machine of any sort -- versus by hand. 80-120-grits are considered medium, and used primarily for smoothing of a surface and/or removing smaller imperfections and marks.150-180-grit are considered fine and good 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. 800-2000-grits are designated ultra-fine and are usually reserved for final color sanding on paint or for final finishing of metal that's destined to remain un-coated. Okay, so now that we've covered the basics of grit, now 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 they discs or paper). Industrial grade sandpaper uses higher quality (read that heavy-duty) components as well as tighter manufacturing tolerances (read that harder and sharper grit that are 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
Metal workers 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 low, a courser grit, and then work your way up to a finer grit. There are also different types of sandpaper, specifically, or more commonly, Open Coat, and Closed Coat. Open-coat sandpaper has gaps and open spaces between the grits which help prevent clogging by giving the removed material a 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 does clog 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 as to our sandpaper and sanding disc choices.

SOURCE
Wet Wedge
Irvine
CA
949-295-5824
www.wetwedge.com
Summit Racing Equipment
PO Box 909
Akron
OH  44398
800-230-3030
330-630-0240
www.summitracing.com