This comes up every once in a while, and last year we had a granite installer sub eat almost $1k for a small scratch on a large picture window which the owner would have been happy with if the scratch could simply be polished out.
Today, with a little snow on the ground, I spent the afternoon polishing granite tiles with a wet grinder and wondered why the same setup couldn’t be used on glass.
In the name of science my work pickup was sacrificed, like a virgin to the carpentry gods. The truck had the passenger side window rolled down repeatadly with a lot of mud on the outside, which badly scratched the glass from top to bottom across probably 1/3 the surface area.
The grinder is a Flex 3,700 rpm water feed with a 5″ rigid backing pad. The two velcro attached polishing pads were the “buff”, which is about 10,000 grit, and an 8,500 grit.
The buff pad, ran flat over the window in wide circles with fairly light pressure for almost a minute reduced the sharp edges in the glass, but didn’t eliminate any of the scratches, even the most shallow. In late afternoon light, after wiping the window dry there was one very light circular scratch, that might have been from an imbeded piece of granite in the pad, or some dirt on the window surface, or it might have been there all along. Other than that, the window was optically no more or less clear in the test area.
Then the 8,500 grit pad was used with firm pressure over a much smaller area for about a minute. After wiping the area dry, there were maybe 10 faint circular scratches, but the test area apeared to be optically no different than the rest of the window. The most faint original scratches were starting to disapear, and the depth of the others were more shallow as well. In better light it will be interesting to see if the glass is cloudy at all.
At least for this truck window, it appears that a stone polisher works well with glass.
FYI
Cheers,
Don
Replies
Don: I work w/ glass. Problem polishing it is that there is no back side to it. You always see both sides, hence you see the finest scratches imaginable.
You can get it pretty smooth w/ the granite polishers, but - to finish glass, you use a thick cerium oxide slurry on a felt pad. The penultimate (that's Brit talk for next to last) step is usually pumice on a cork pad. There is a another polishing pad w/ very fine diamond that is starting to replace the cork. I have one, but don't recall what grit it is.Trust me, it's tough to get the original scratches out, but if you can, get a felt & cerium oxide setup, and you can make it look as good as new.
Don
The Glass Masterworks
"If it scratches, I etch it!"
Don, Thanks for the explanation.
Do you just use cerium oxide or do you finish with that after having run through a series of different abrasives. What rpm do you run the felt pad at?Karl
Karl: Depends on how bad the scratches are. I do my work on a 36" dia lap plate. Runs at a couple hundred RPM. I run through 5 different grades of grit. Start at 100, 200, 400, 600, felt w/ cerum oxide. I'm not in my shop, so I cannot look at the pads. I just have them stacked up & use them in proper sequence. I think there is a pad between the 600 & the felt. For a car windshield, there is a polishing kit that has all the appropriate stuff in it. You say you have the grinding wheels for doing granite. They are probably diamond. Granite & glass are same hardness, so you can use same tools on it. Gotta be careful that you don't get larger grit contamination on wheels - but you already know that! All the same principles as grinding & polishing granite apply to doing glass. You can do one, you can do the other. You don't want to run too fast - the last thing you want to do is heat the glass from friction because you dry out the pad. Takes a fair amount of pressure to do the cerium polishing. The secret is to ensure that as you finish each grade of grit that there are no scratches from the previous grade left behind. When you are ready to use the cerium, the glass should have no scratches/wheel marks at all. You cannot remove anything w/ cerium. The glass should be just hazy at this point. All the cerium does is polish out the haze.Good luck.DonDon Reinhard
The Glass Masterworks
"If it scratches, I etch it!"