What’s in your abrasive?

At the risk of sounding like one of Monty Python’s Northerners (Now we had it tough!) we are spoiled by the range of abrasive papers we have available to us – and often reasonably priced too. I remember being given badly torn quarter sheets of glass paper in school woodwork lessons and being expected to make them last and also produce a smooth finish on whatever useless object we were making at the time. Not only did the sanding effect of the glass paper not last long, but rogue highpoints of glass particles on the paper also produced scratches that were even more difficult to
remove.

Glass paper may not be quite as bad as using dried sharkskin or crushed shells glued onto parchment as happened in 13th century China, but it suffered from the problems of a sheet-based abrasive – namely it had a short sanding life, the glues that held it onto the backing sheets were not good enough and the backing sheets themselves were either too soft or too hard to do a good job.

The introduction of power sanders into a growing DIY and professional market forced the development of better abrasive sheets. I do not remember my first Black and Decker orbital sander attachment with any degree of fondness – It was a great way of destroying the bearings of the drill without actually getting a much better sanding result. But I was introduced to garnet paper – a quartzy sort of stone coating that seemed to last longer than glass paper and gave a marginally better finish.

In my own mind, the development of sanding machines and the required sanding sheets has been a steady improvement since the 1970s. I still do a lot of sanding on various jobs, but almost every time I do it I feel grateful for the tough and long lasting abrasives, hook and loop fixings and good dust extraction. Jobs that used to take hours now take much less time and the results are so much better.

Key developments in the ‘Sanding Revolution’ for many trades, in my opinion, are the invention of the random orbit sander, harder abrasive materials, better adhesives and electrostatic techniques for applying the abrasive to the backing sheets. These all add up to making sanding much less of a chore and make for good results by most users – provided they make the right choices…

Raw Materials – Abrasive Agents

Most abrasive papers commonly used by tradespeople (not industrial abrasive applications) use naturally occurring minerals like aluminium oxides, corundum and silicon carbides. I won’t be too concerned about diamond or boron or other synthetic derivatives because they are used in very specialist applications – and have the associated extra costs attached.

Most users will buy aluminium oxide based abrasives from their usual outlet/store/online or whatever – although the colours of the abrasive may be anything from a brown, a red, a yellow or even a white colour. These colours may or may not have any significance for the abrasive itself, depending on the manufacturer. In truth, aluminium oxide is a very good material because it is versatile and does not take that much preparation for its abrasive role. It is made from aluminium ore called bauxite, and is generally baked at a high temperature and then crushed to the right grade of abrasive particle. The smaller the number (e.g. 40 or 60), the bigger the grit size and the rougher and faster the cut. Higher numbers like 240 or 320 indicate a much finer grain size and papers that would be suitable for fine finishing for a very smooth surface.

The Papers – or Backings

Most modern abrasive sheets do not have paper backings so the term ‘sandpaper’ is out of date on two counts. Since most current abrasives will be attached to a block or to a sanding machine, the backings need to be strong and very flexible – therefore most backings are based on a strong fabric. There are still some light duty abrasives that use paper backings, but these cannot be used where the abrasive might get wet since both adhesive and paper will slowly disintegrate.

On a belt sander, for example, the backing of the sanding belt probably gets a harder time than the abrasive itself because it runs quickly between the drive and the idle roller, generating some heat and downward pressure on the workpiece. Here you can easily see that the backing material needs to be strong, flexible and hardwearing. Experience has taught me that cheap sanding belts are generally not good value either because the abrasive wears too quickly or the belt splice breaks.

In the old days (there I go again!) most sanding sheets were held onto the machine sander with spring clips but the invention of hook and loop fixings have mercifully made this method pretty well obsolete. It is simply so easy to peel off an old sheet and apply a new one, that no one can be forgiven for not ‘going through the grades’ (from rough to medium to smooth grades) when sanding a woodworking project, for example. The hook and loop fixings are also so secure that sander bases and sanding sheets can be perforated to allow vacuum dust extraction. Some of the modern dust extraction systems are so good that being covered in dust while sanding is simply no longer necessary. (And of course, not desirable)

Applying the Abrasive – Not so Easy

The key to our fast-cutting and relatively scratch-free abrasive coatings has a lot to do with the development of electrostatic methods of applying the abrasive to the backings. In the past, and currently some cheap abrasives too, were made by simply having a hopper full of abrasive powder falling through a ‘sieve’ onto a continuous belt of already glued backing. With no control over the orientation of the grains the random abrasive grains could create high and low points. The high points often made deep scratches and the low points were largely ineffective at sanding. The grains would also cut better in one direction – hence the arrows on a belt for a belt sander that helped get better grain direction for fast cutting as well as protecting the splice joint on the belt. Nowadays most of the arrows on sanding belts can be pretty well ignored because of electrostatic coating. Using this method, the abrasive grains are electrically charged and during manufacture are drawn up from the hopper to the oppositely charged backing. This orientates the particles so that they stand upright in the backing adhesive, thus eliminating high and low spots. So, no scratching and ineffective low spots. Also, as the abrasive wears, the points break off, leaving other abrasive points that continue to work and effectively extend the life of the abrasive quite considerably.

As usual the application of knowledge, experience, understanding and technology have resulted in a range of abrasive products that I would have dreamt about in the early 1970s. Like Monty Python’s Northerners, when it comes to ‘sandpaper’ the Old Days were ‘tough’ and I am happy to see the back of them.