Norton MultiAir A975 Speed Grip
In a well-known auto painter time study, it was determined that those in the paint department spent one-third of their time at work sanding something. Metal shop techs are also frequently abrasively scrubbing away at the vehicle for one reason or another. While sandpaper may be a mundane item, it’s clearly a big part of what happens in collision repair. It is, in fact, a multi-point cutting tool designed to penetrate the substrate and pull the chipped material out of the scratch, leveling the old finish and providing increased surface area to improve the adhesion of the new coatings.
Automotive refinish sandpapers are manufactured all over the world. From Mexico to Portugal to Finland to Canada to the good ol’ USA, numerous manufacturers produce quality goods to expedite the refinishing process.
An educated consumer will understand the basics of something before they can appreciate the features (the unique characteristics of that brand), which lead to the advantages (better than the other choices because…) and finally the benefits (why your job will be easier, faster or better) of any particular brand. I’ll leave it to each vendor to convince you of their features and benefits, but will endeavor to inform you about the construction of automotive grade sandpapers.
From the Bottom Up
All abrasive papers and cloths begin with the backing material, on which is applied the first coat of adhesive, into which the abrasive particles are distributed. After their application, a second coat of adhesive is applied to hold the abrasive minerals upright on the backing and minimize clogging.
The residue from sanding is composed of both the abrasive minerals breaking down into dust and the sanded surface being removed. This mixture is correctly called “swarf.” Think of it this way: wet sanding provides the truest cut because the sheeting water washes the swarf out of the way of the next cut, preventing the clogging of the remaining abrasive minerals with hot, melted filler, primer, paint or clear. With the swarf out of the way, any sandpaper will last longer.
Most automotive refinishing sandpaper has a paper backing, although some types use foam or plastic film. Plastic film is an important development for waterborne/low-VOC base-coats, says Anne Knight of Carborundum Abrasives North America.
“Waterborne technology allows for thinner coats of paint, so it’s critical not to cut through them,” said Knight. “So if the substrate is totally flat (like film), then the ‘highs and lows’ of the grains are minimized and you get a more consistent cut, especially when using a tighter grain sizing.”
Heavy-duty abrasives can use a resin fiber backing, which is multiple layers of specially impregnated paper, and still others use cloth in various weights and stiffnesses to back the abrasive minerals.
Ordinary paper and cloth backings use a letter to delineate their thickness, strength and flexibility. By weighing a ream (480 sheets) of 24-inch by 36-inch paper, a weight in pounds is established and a letter is assigned. The lightest weight automotive paper backing is assigned the letter “A,” and heavier weights of thicker papers are rated “B,” “C,” “D,” “E” and “F.” In each case, the backing is made from heavier, thicker and more durable paper.
More durable backing papers are used with more durable and long-lasting abrasive particles. Putting a ceramic abrasive particle on an “A” weight paper would cause the paper to give out before the mineral was exhausted. Likewise, putting an inexpensive silicon carbide particle on “E” weight paper would be foolish, as the paper would outlast the mineral.
However, some manufacturers claim to have gotten around this paper weight-abrasive type issue.
“We offer a B-weight paper that’s reinforced with latex fibers to make it incredibly strong and tear-resistant, and that’s what we put our ceramic grain on…which is an exceedingly durable abrasive,” said Carborundum’s Knight. “Components can be added and formulations can be modified to enhance the strength and characteristic of a backing.”
Cloth backings are commonly used in grinding belts and are rated by their weight and flexibility. “J” weight cloth backing is very flexible and typically used in polishing operations. “X” weight cloth is heavier, stronger and less flexible, and “Y” weight cloth is extra heavy duty and used in the heavy grinding of bare metals.
Download a Sandpaper Spec Chart