It seems there is some confusion out there about sandpapers and the properties of the different abrasives. After this column ran, I received two letters from people who said I had my information all backwards. Both of these people had worked with abrasives for years and thought they would set me straight.
Unfortunately, I had to send them the research materials I had from some of the top abrasive / sandpaper manufacturers in the world. I hate dishing up humble pie, but I had to in this case.
The reason I am bringing it up is to tell you that each and every day I find workers who unknowingly are either spreading inaccurate information or are possibly using the wrong tool for the job. This can impact you if you decide to hire a worker who thinks they know what they are doing simply because they have been doing it for 15 or 20 years. The sad fact may be that they have been doing it wrong for that length of time and never realized it!
Some Other Facts
Just after the column ran in a Kentucky newspaper I received a phone call from a reader who worked in a factory. She sanded fiberglass products each day and often saw the words "Open" and "Closed" on the back of the sanding sheets. She asked me what that meant. I told her it was an excellent question.
The abrasive particles in sandpaper can be distributed onto the paper in one of two ways. You can completely cover the paper with grit or you can apply it so that there is space between the grit particles.
Open coat sandpaper has space between the particles. The paper surface often only has 40 to 70 percent coverage of grit in open faced sandpapers. This property is really necessary when you work with raw wood as this space prevents the sandpaper from clogging with dust.
Closed coat papers have no place for the dust to go. For this reason, they are ideal when working with metals that produce an ultra-fine dust.
When you go to buy grit, you might get confused. The coarse papers have small numbers like 40 or 60 and the fine sandpapers have large numbers like 180, 220 or 240. You can even get industrial papers that have numbers as high as 1,600!
The numbering system makes sense when you think of how the grit particles are separated to make the sandpaper. The numbers correspond to the number of holes per square inch in the mesh in the separating sieves used to filter the grit as it is crushed. If you want to get 220 holes per square inch, this means they have to be small. See how it works?