Our techniques for Polishing Glass with Mirka Abranet SIC NS and Polarshine E3 Glass Polishing Compound

At our shop, we routinely have customers with glass issues that require repair rather than replacement. Some of the repair versus replacement decisions are due to cost and some are due to the fact that the glass simply cannot easily be replaced. For instance, we deal a lot with antique windscreens, and the customer desires to keep the original glass. We also have customers with yachts that have developed scratches on some of their large window glass. This glass is all but impossible to replace, and is therefore much easier repaired in place. Therefore, we have developed a method for glass polishing to restore, rather than replace, dull, scratched or nicked glass. This process has served us well over the years.

There always is one or two major bothersome scratches that a customer will notice, but on the more micro level, smaller less noticeable scratches and nicks refract the light for an overall defused and lifeless look, and even often a distorted image. Often small ridges, scratches, and pits in the glass surface hold and harbor dirt, oils and residues that make for a murky surface. This damaged or lusterless glass becomes even more noticeable when we varnish or polish the hull, or spruce up other interior components. In other words, we inadvertently make the glass look worse by making the rest of the boat look better. To overcome this phenomenon, this is the procedure that we use in our shop to bring damaged or lusterless glass back to life:

CLEAN, GRIND, POLISH, AND COMPOUND THE GLASS

The first step is to choose an abrasive media that will cut the glass and take out the scratch. There is little argument that Silicone Carbide is the abrasive of choice for cutting and polishing glass in every respect. Unlike Aluminum Oxide, the Silicone Carbide does not dull, but instead it breaks away in cubes or squares leaving another sharp cutting edge in its place. As such, Silicone Carbide, you might say, cuts more like a machine tool blade than a piece of sandpaper. This cutting and slicing effect is what is needed for cutting glass for scratch removal and for generally renewing the surface of the glass.

Polishing glass is not complicated, but it takes some patience and it takes time to develop a feel for it. To start with, everything must be right. The right abrasive, the right pressure, the right surface temperature of the glass (not to warm or too cool), and just the right amount of lubricant, are just a few criteria that you need to develop a feel for the polishing process. Practice is everything. Once your abrasive is cutting the glass instead of just spinning in place, you will feel the difference, and you are on your way. It is hard to articulate exactly what you will feel, but it is noticeable. Perhaps the best description is that you will feel it start to grab just a little.

The correct choice of tool is also important, but again, there is no right answer for everyone. A rotary grinder polisher set to a low speed is without a doubt the fastest tool, but until you are proficient, that tool may get you into trouble. Your DEROS electric sander with its orbital component, and the lack of forced rotation, is slower, but is a very safe bet. Putting a 5-inch pad on your DEROS will work the best. It will undoubtedly take longer than a forced rotation rotary or orbital polisher, but it may save you the time of making an irreversible mistake. So take it slow, use a safe tool like your DEROS, with a 5-inch pad, and you should be fine.

If you own a 6-inch DEROS, no worries, Mirka makes a 5-inch, 130 gram backing pad, part number 915-GV20130G, that will allow you to use 5-inch abrasives on a 6-inch DEROS without compromising the balance. You can find it at Get-Mirka.com.

But…before you can cut and grind, you must clean the glass.

CLEANING THE GLASS

Debris on the glass laying trapped in the fine scratches and pits also can foil the polishing process by affecting how the abrasive does or doesn’t grab, and get traction for cutting on the glass surface. That dirt and debris, especially oily debris may cause your abrasive to just slide over the glass surface without cutting or even just compromise the cutting process. It is a must to clean the glass very well before starting.

Soap and water, followed by Windex has worked well for us, and/or depending on the type of grime, we also sometimes use denatured alcohol. Once the glass is clean, we give it a quick scrub with Mirka Polarshine E3. The E3 is not being used as part of a polishing process at this point, it is more like using a hand-soap with Pumice in it when it is used for cleaning. Polarshine E3 is a water-based polish, so it is great for cleaning, because it is, itself, easy to clean back off the glass. The E3 watered down does an excellent job cleaning and giving the glass a light scrubbing.

A lot of professionals use this cleaning trick and spread the E3 with a 5-inch felt pad, the same that would be used for polishing. We have found, however, that if your piece of glass is murky and dirty then using the watered down Polarshine E3 with a piece of 1000 grit Abralon instead of a felt pad works even better to clean the glass. The Mirka Abralon is a great scrubbing tool to grind out the grime and debris.

Once the glass has been scrubbed clean with the Abralon and Polarshine E3, then it is washed back off again with water and detergent and/or Windex, and wiped down well with a clean microfiber cloth. Rinse and wring out the microfiber cloth often, or replace as necessary, as you go so that you don’t just end up smearing the grime around, cleaning one place and then just moving the same grime to another. At this point, the glass should already feel cleaner and less slick. It already will probably look a little better, but there is a lot more to do.

CUTTING GRINDING AND ABRADING THE GLASS

But now it is time to get to work cutting or grinding the glass. For cutting down glass, no other abrasive beats Silicone Carbide as your choice of an abrasive media. This is where Mirka Abranet SIC NS really shines. Abranet SIC NS is the best of all worlds when working with glass. It is Silicone Carbide (SIC), and it has no Stearates (NS). Stearates are used to effectively lubricate the surface of Abrasive Papers to help keep them from clogging. In the case of glass, clogging is not an issue, and the lubrication from the Stearates would keep the abrasive media from properly grabbing the glass. The combination of the Silicone Carbide and lack of Stearates make Abranet SIC NS the perfect Abrasive for glass.

When working with glass, you will need to apply all of your good sanding habits, no different than working with wood, paint, metal or solid surface. So for a quick recap of the most important sanding habits…

  • Don’t start the machine until it is flat and squarely on the glass.

  • When it is time to stop, pull the sander away from the surface of the glass quickly and concisely before you stop the machine completely.

  • When you are cutting, work with a consistent pattern, occasionally feathering in the edges as you go.

  • Use the sander completely flat, and never try to speed up the process by lifting the sanding pad on an angle or by trying to use only the edge of the pad to cut. Such poor sanding practices will leave you with an uneven surface fraught with divots, and the dreaded half moons.

  • If you are dealing with curved glass, then use of a firm interface pad helps with the curves.

Abranet SIC with its Silicone Carbide abrasive cuts very differently than its Aluminum Oxide cousin. Aluminum Oxide abrades until it dulls out, and produces a finer dust or chip that is easily extracted by the vacuum. Silicone Carbide has a property known as “friability.” Friability is a phenomenon where as the old material breaks away, the cubic pattern produces a brand new sharp corner and edge, and it starts cutting again as if it were a new piece of abrasive. So Abranet SIC NS cuts the same from the second you put on a new sheet, until the Abrasive grains are gone and the piece of abrasive is completely spent.

The chips produced when cutting glass with Silicone Carbide are more like the chips produced from cutting steel with a blade or a cutting tool. They are bigger, more consistent, and the Vacuum doesn’t as easily extract them. This is another reason why the net component of Abranet SIC is perfect for glass. The net construction of Mirka Abranet very literally gives a pocket to temporarily hold the bigger glass chips that are not quickly extracted. Therefore, every so often simply lift the DEROS from the surface of the glass with the machine still spinning and you will see the bigger chips fly out centrifugally. (You will be glad you wore your mask and your eye protection). After the chips fly out, stop the sander, rest it back squarely on the glass surface, and start sanding again. The soft start and rapid stop features of the Mirka DEROS will give you the perfect start and stop function for this operation.

If you feel your experience allows you to use a grinder/polisher instead of your DEROS then the procedure is the same, except you won’t get the benefit of the soft start and all of the other features of the DEROS. On smaller pieces we still always gravitate to the DEROS for cutting and polishing glass. It is noticeably slower than a grinder or a polisher, but on a small piece that time difference is not so significant and the DEROS is so much lighter, easier and nicer to use and definitely more accurate and controllable.

Keep in mind that all Abranet products are designed to work best with a light touch, and that is all the more true with Silicone Carbide. Pushing too hard just prematurely breaks the Silicone Carbide grains and you will just be wasting it. Just a light touch and let the Abranet do the work for you.

CUTTING AND GRINDING THE GLASS

So now it is time to start grinding some Glass. Good woodworkers have all been taught not to skip grits, but because silicone carbide cuts more like a blade we have found that you can most certainly get away with skipping grits. So the next question is where to start and what grits to use.

For bad scratches, Abranet SIC 80 is a good place to start followed by 120 grit. Where the scratches are not so noticeable, or where the problem was more like just pitting, then starting with 120 should work fine. I have found that you don’t need to be a scared of the 80 grit with glass as you would, for instance, with wood. Because Silicone Carbide cuts more like a blade than a stone, the lower grit does not seem to really cut deeper or “rougher,” just faster. Just watch your work and your progress closely, and you can control it like a pro so long as you remember to use a consistent pattern and stop from time to time to blend and feather in your work area into the surrounding areas so you don’t get a low spot or patches with differing refraction.

I travel about the same speed that I use a paint gun, with the same 50% overlap pattern. I go up and down in a small area, then left and right in the same area, then feather that in and move on to another area with a 50% overlap on my whole pattern. Many people may develop different patterns of their own, and that is OK, but slow and consistent is the main key. Overlap and stopping often to feather in your work area is equally important. Your first cut is easy, and you can see where you have been. After the first cut, it’s not so easy to see, and the only thing you can rely on is the consistency of your pattern.

Do not ever stop your motion or your pattern in any one spot, instead, keep the sander moving.  The DEROS works well at 4000 RPM, but if you use a grinder or a polisher, then 2500 RPM is more ideal. If you use an Orbital Polisher, such as your Rupes Polisher, or a Flex Polisher, then use the lowest setting available. If you do use something really heavy like your Rupes or Flex Polisher, or certainly if you use something as heavy as a Festool Rotex and you are working horizontally, DO NOT PUSH the weight of your machine is heavy enough. Those types of machines are almost too heavy as it is. In fact, a light upward pressure may end up being more appropriate when using a heavy sander horizontally.   Steady surface pressure is the key to cutting consistently across the whole surface of the piece of glass. Light to moderate pressure wins the day.

So to reiterate:

  • There is no benefit to pushing hard, it will prematurely shatter the silicone carbide, and all you will do is wear it out faster.
  • If you feel it cutting and if you see the chips, then you are pushing hard enough.
  • Go slow, relax, and stay consistent.
  • Use the same good posture and body mechanics as you would use for sanding or plaining.
  • Keep your pressure as even all across the surface of the glass, and never use heavy pressure in the center.

Abrade the entire work area first to establish clear boundaries, and this approach also will bring out the scratches. If you work on any single scratch, blend and feather out the area often to establish evenness as you go so that you don’t end up with refraction distortions caused by a grossly irregular surface.

I work my way through the grits with the Abranet SIC NS, starting with 80, then 120, then 240, then 400, thoroughly cleaning the surface with a microfiber cloth between girts. By the time you get to 400 grit, hopefully all of the scratches are gone, and the cutting is complete. I always like to finish my last Abranet SIC pattern with an overlapping circular pattern just to give one last chance for everything to blend together nice and flat, and consistent.

POLISHING THE GLASS

Now that the cutting phase with the Abranet SIC NS is complete, it is time to start the first polishing process using Mirka Abralon. Mirka Abralon is also a Silicone Carbide Abrasive, but with Abralon, the Silicone Carbide grains are bonded to a flexible knitted backing. Therefore, the Silicone Carbide does not cut like it does when bonded to Abranet’s more rigid mesh fiber structure. The flexible knitted fabric conforms around the surface profile, and maintains that profile while it polishes. So, if your glass surface is not flat and scratch free yet, with a nice even refraction, it probably means you rushed your way through the cutting process. Patience is the most important ingredient in this sanding and polishing process. If your surface is not where you want it, you may as well go back and fix it now, because you are going to need do it sooner or later. Rough polishing with Abralon is all about achieving the desired mirror like surface refraction. This type of polishing will not further flatten the surface or remove any significant scratches or imperfections. If you are not flat and scratch free at this point, you are best to go back to Abranet SIC, drop back a few grits, and go slower and steadier this time. To reiterate, patience is the key. Only if you have an even haze, and an even refraction, you are ready for Mirka Abralon.

A lot of excellent tradesmen would go right to Abralon 500 at this point, but our experience is that when you switch from Abranet to Abralon in many different applications, it is best to drop down a grit. The scratch patterns are just different. Even 500 Abralon will not sufficiently remove the scratches left by 400 Abranet, and you will need to drop down a girt. We apply that same procedure with glass. After the 400 Abranet SIC NS, we go to 360 Abralon, and start working our way up from there. We still clean thoroughly with a microfiber cloth between grits.

Once you are using the Abralon, it is time to start monitoring temperatures of the glass surface. Since both minimum and maximum temperature are important factors to proper performance and achieving perfection with Abralon, you will need to work in a small area to prevent the heat that you generated from the friction of the abrasive from bleeding away. A hand held infrared thermometer is ideal for monitoring the surface temperature. Check the temperature of the glass often. If the glass it to cold, the Abralon will not polish effectively, it won’t “grab,” and it will just slide over the surface. With some practice, you will feel when enough friction brings the glass to temperature, and the pad begins to cut and polish.  Too much lubrication at this point will counteract the friction necessary to bring the temperature up to where you want it. Additionally, if you try and work on too big of an area, the glass will not maintain appropriate temperature, either. It takes some practice to find the perfect sweet spots with lubrication, surface area, and temperature.

Conversely, if the glass becomes too warm, this can create problems, including damage to the glass. From our experience, we use 160 degrees Fahrenheit as our maximum when using Abralon on glass, and this temperature has served us well over the broad spectrum of different glass types. As discussed previously, it also is important to keep the work area small in order to best control the temperature.

Spritzing the Abralon with water also works well to both lower the temperature and to provide lubrication. However, you must be careful to find the correct balance, because too applying too much water will cause to Abralon to stop cutting. It takes practice and experience to get it just right. So remember the key points here:

  • Monitor Temperature.

  • Allow the glass to come up to temperature for the Abralon to start cutting.

  • Work in small areas to control heat.

  • Do not allow the glass to exceed 160 degrees Fahrenheit

  • Lubricate often, but best to error on the dry side.

We often use the 360 completely dry, and then move up to the 500 Abralon, also dry. We will then start spritzing it with water or Windex when we move up to the 1000 Abralon.

Every now and then you can wipe off the glass with a microfiber cloth to see what scratches you have missed, but the color of the haze is a good indicator that you have cut evenly. You certainly should not see any dark white lines or patches that are more clear or shiny than others.

So in review, we finished our last of the grinding and cutting stage with 400 Abranet.   After the 400 Abranet we switch to Abralon, starting with the 360 grit dry, going to the 500 dry until you see only a light but even cloud on the glass. Really take your time with each of these steps, stopping from time to time to clean the glass. When you achieve only the very light even cloud on the glass, you can move to the Abralon 1000 wet.

Now to Abralon 1000 wet, and this gets a little tricky, but it is just a matter of practice. Just a little too much water and the Abralon will stop cutting. But you can feel a little drag on the pad and it will help you get to the sweet spot. Temperature is also an important factor. Use your slow and consistent pattern being very patient. I use a pattern that is up and down, overlapping 50 percent, then switch to back and forth, also overlapping 50 percent. I slowly run that pattern 4 times in each direction on each section, and then switch to a once over slow circular pattern that goes outside my work area for the purpose of blending before I move on to the next section. When that whole procedure is complete, 4 passes up and down and 4 passes left to right, and blending with a circular pattern, only then do I wipe with a microfiber cloth and move on to 2000 Abralon. I then do the whole process over again with the 3000 Abralon. It is as tedious as it sounds, but it is very much worth the effort in the end.

COMPOUND AND POLISH

Now that you are finished with the Abralon, it is time to start polishing with compound. Before you start polishing, it is very important that you completely clean the glass of any debris from cutting. Even a single chip and it could just scratch your glass all over again. Because there will not be any more chips generated, I take a little extra time and give the glass a very good cleaning with water and Windex before I start compounding, wiping the surface thoroughly as I go.

Now, we are ready to start compounding. Because it does not take much E3 to do the job, it is good to first completely coat the work area with a thin coat of polish. Use the same felt pad to accomplish this application. This practice not only assures that there will be no dry spots, but the haze from the compound acts as an indicator of your progress and where you have been. We use the Mirka MPADFP-5 Felt Pad. We find that the Mirka MPADFP-5 felt pad is one of the finest felt pads in the industry.

We use an orbital polisher. If you are not an expert polisher, then orbital is absolutely the way to go. For larger jobs, we use the 5-inch Rupes with a 15mm orbit. For smaller jobs we use the 3-inch Mirka Polisher with a 12mm orbit. If you do not have a professional orbital polisher then you can still use your DEROS with its 5mm orbit even for the polishing stage. The DEROS will work just fine, but it will take a little longer. A very small sprits of water on your felt pad helps glide it along as you work

Move the polisher at about the same speed as you would move a spray gun when painting, and with that same analogy, overlapping each pass by 50%. I usually use the 4 and 4 rule. The E3 works very quickly at making the glass really look great, but don’t be fooled into cutting corners at this point, follow through completely with your pattern. The felt pad may need a spritz of water from time to time, but be careful not to overdo it. If water is slinging out the side, you have used way too much, are likely not polishing anything, and are just sloshing around the glass.

The Mirka Polarshine E3 on the MPADFP-5 felt pad still will generate enough friction to raise the surface temperature of the glass. Therefore, it is still important to monitor your surface temperatures when using the Polarshine E3 and the felt pad. As a good rule of thumb, if you are not generating heat, then you are not polishing. With the E3 and the felt pad, as a rule, we use 120 degrees Fahrenheit as a maximum temperature for this part of the process.

When the polishing is complete, I go one last polishing pattern, this time small circles with an almost dry pad. Polarshine E3, like all polarshine products is water soluble. This makes clean up easy. You can give a final cleaning to the glass with warm soapy water followed by a cleanup with Windex. By now, hopefully you are looking through a beautifully polished piece of perfect glass with a flat crystal refraction.