I could literally write a small book on this topic! Simply put there is a tremendous difference in the chemical makeup of hard water stains. However, as I am going to demonstrate, it is the difference in the surface they are on that determines mostly the reason why some are much more difficult to remove, while others might look worse but are much easier to remove.
Most HW stains are based on a blend of different silicates and silica. Which are based on elements like silicon, oxygen, iron, copper, aluminum, and some of the other earth metals including calcium and sodium. Silica is just based on silicon and oxygen. Which takes on different configurations but is still silicate. So essentially different chemistries that are very similar to glass itself. This is the basic reason why most HW stains are so difficult to remove from glass. Especially dark tinted glass like brown or blue.
When HW stains are based on silicates and silica the only acids that will dissolve them are the same acids that dissolve/react with glass. If you attempt to remove silicon based stains with an acid like acetic or HCL you will not have much success if any. The only way the glass can be restored is through either a one step polishing process, or a double or even a triple grinding and polishing process. The grinding process can be done dry or wet, and produces an even milky opaque surface. Which can be brought back to full transparency with a cerium oxide slurry or compound. This is a technique that must be learned. The same process is used for removing scratches. Silicon carbide is used for the grinding process.
As Marc Tanner says down in gatorland every window is different. This is simply because every stain has a different fingerprint. It is both chemically and physically different. And because of this the stain itself can and usually does break up during the grinding and even the polishing stages. When it breaks up those particles as much as 100 microns each and greater will create what Marc calls the spirograph effect. Which can also be caused by incorrectly mixing the polishing slurry. Cerium particles have the tendency to agglomerate if the cerium is put into the container first before the water and then mixed.
The grinding and polishing techniques both need to be finely tuned. Also the ability to accurately observe the quality of the "new" surface that is being created is equally important.
I feel it is most necessary to bring up the point that the physical and chemical properties of the surface worked on will determine the greater difference in how easy it will be to remove the stain. Whatever the properties or chemical identity of that stain happens to be. I learned this fact from one of my consulting jobs in Chelmsford Mass. The building had all of the lower level windows covered with hard water stains from the sprinkler system. It also had two types of window surfaces. Both dark glass the color brown. Also a metallic low e coating that was slightly reflective but plainly of a different chemistry and was more smooth. But the stain was the same exact chemistry for every window because the water source was exactly the same for every sprinkler head. The dark glass was a total bear to clear with a one step cerium method. But the low e windows were exceptionally easy to clear even by hand. No machine needed!
Frictive surfaces will always hold onto stains with an incredible tenacity. Whether such surfaces were converted with an acid such as HF, sulfuric, or an ammonium bifluoride. Or whether they were converted with a polishing compound such as cerium oxide. Both of these are commonly used to remove stains. Every time they are used the surface is converted to a frictive surface. You can actually feel this with a dry fingertip. But you might not know if someone was there before you to remove the stain. So you might have an easy time removing the stain from most of the windows on the building, but then come to the last floor which is next to impossible to clear. It might be so difficult that the same exact one step polishing method you were using is now absolutely impossible. Making it necessary to also use a silicon grinding method as a precursor to polishing with a cerium.
I am a firm believer in wet compounds based on high quality super-abrasives. Whether these are based on cerium oxide, silica, silicon carbide, aluminum oxide, polymers, diamond, or even hybrid blends. They can be converted to slurries right on the window and applied with a syringe.
Written by Henry Grover Jr.
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