Building A Super-abrasive Compound For Restoring Coated GlassWritten by Henry Grover Jr.
What are Super Abrasive Compounds?
SACs are Polishing Compounds which are based on a homogenous mixture of certain super-abrasive dry powders and a thick liquid carrier. SACs have the same proportion of their components throughout any given sample. They can be made with a rather low powder density or a high powder density.
What Carrier Liquids Can Be Used?
My favorite is glycerin. It is very simple to use. It also goes quickly into water so it is easy to work with. Or we could use polyethylene glycol which also goes easily into water. Another alternative is to first thicken some water with guar gum, locust bean gum, or xanthan gum and then mix this with either glycerin and or polyethylene glycol (PEG). It is easiest to add the super abrasive powder to the thickened water first, then add the glycerin or PEG after. Use a blending machine. I also like to dissolve some citric acid crystals in the water before thickening to adjust the pH so as to prevent bacteria from forming if the end product will be doing some time on a shelf.
What Exactly Are Super-abrasives?
These come as powders. Which are based on microscopic or even nanoscopic particles. I am most familiar with cerium oxide, microcrystalline silica, aluminum oxide, diamond, and zirconium oxide. Most Window Cleaners use cerium oxide. The highest quality is white. Other colors such as pale orange are due to impurities. Cerium oxide is great for polishing glass because it has mechanochemical properties when used on this surface. Here we are talking about coated glass which surface is usually titanium dioxide. As for optical grade microcrystalline silica, this can be exceptionally ineffective or very good depending on the product. It definitely has it's place because it is half the price of cerium oxide. Even now since the cost of cerium has come back down somewhat. Aluminum oxide is rather ineffective most of the time but definitely has it's place for different technical reasons which I won't get into here. Diamond is a tricky item. It is difficult to control with a machine. Easily causing scratches. But when used correctly it will do things that all the other super-abrasives cannot. It is also a great powder to add to cerium or microcrystalline silica just to kick either of these up a notch! Zirconium oxide is a good powder to add with cerium and will work by itself just fine.
How Important is Particle Size When Restoring Coated Glass?
Size is very important in my opinion. My preference is 3.5 microns average particle size. However this should not be your only consideration. When looking at the tech sheet for any product/powder you need to pay attention to what is called the Particle Size Distribution or PSD. Very simply put this is the percentage of particles that fall within the average size range. It can be graphed. You want the tightest or most narrow curve. PSD affects the quality of the finished surface. Also how fast you can remove stock. Because a very narrow PSD allows for many more contact points which results in a much more uniform and smooth finish/surface. If using a machine with a felt pad on a coated surface I suggest moving slow. Most guys spin at 1200 rpms when on glass but coated surfaces are much more sensitive to scratching. I have even heard of a company using a nano-scopic aluminum oxide on a damaged silver coating, but at a high rpm. This was an exceptional example of what a "lesser effective" aluminum oxide can accomplish. The coating had been "weakened" with hydrofluoric acid, such that cerium was literally stripping it off the window. Typically stains are much easier to remove from coated surfaces than clear glass or dark tinted glass. So heavily aggressive polishing techniques are not required.
What of Particle Hardness and Shape?
If the particle is softer than the surface you are working on you will not accomplish much. At least if you are attempting to remove mineral deposits from sprinklers or concrete efflorescence. Both glass and coated glass (hard coats) have a hardness of about 7 months. The soft coats are called that for a reason. But are becoming more resistant to abrasion these days. Particles that are harder than coated surfaces can more easily scratch if they bunch up (agglomerate). Shape is important because particles that are amorphous or cubic can have rather sharp edges. These are rather effective at cutting but can also more easily cause scratching. There are other particles out there that are shaped like a ball or flat flakes. Balls roll and flakes slide. So either shape tends to rub rather that cut. The shape or the morphology of the particle is listed on the tech sheet for the superabrasive powder/product you are buying.Continue Reading Below...