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.
Sandpaper is sandpaper, right?...
Sandpaper is sandpaper, right? Not so. Cheap, light-duty sandpaper will often be better at pissing off the user than removing material. Pass on the cheap stuff and stick with professional type industrial-grade paper and discs, like that offered by 3M, Norton, or Carborundum. There’s nothing worse than abrasives that wear out quickly.
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?
Abrasives come in a wide array of designs: 9x11 sheets; 5-, 6-, and 8-inch diameter discs; 4x8 sheets; and 2¼x17½-inch long-board paper are the most common in the autobody trade. All are available in a wide variety of grits, and the 9x11 is the standard for wet or dry paper.
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).
When it comes to wet-sanding,...
When it comes to wet-sanding, the new (at least new to me) sanding blocks from Wet Wedge are ingenious products. They’re designed to save time and sandpaper, plus they go a long way to ensure a perfectly straight finish (even for me, someone known for building the friendliest cars in town, cuz they wave to everyone as they go by).
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.