cabcat: (pleasant)
This came out of a discussion on youtube about wether '0000' grade steel wool could cause damage to non tempered glass.  Here was my response after much looking about, it wasn't a little confusing as terms like tempering are misused.


"A few people have reported scratching using this method, I personally haven't ever cleaned glass with this method so I did a bit of research to understand it all. It's not only confusing as some terms are misused a lot but there's also conflicting information when describing the modern windscreen manufacturing process. Anyway this is what I found. (If you find any decent source please reply as this is now bugging me)

With cars, I'm not sure with RVs, the front windscreens are now, and have been for a long time, made of laminated glass with a couple layers of glass sandwiching a layer of PVB (Polyvinyl Butyral). These glass layers are heated to a lower temperature and slow cooled (annealed) during the manufacturing process. As opposed to super heated then differentially cooled, like with tempered glass.
http://www.pilkington.com/pilkington-information/about+pilkington/education/processing/automotive+shaping+and+strengthening.htm , http://www.nags.com.au/products/laminatedwinscreen.aspx

This basically means glass in laminated windscreens has a lower MOHS rating than the side and rear glass which is usually tempered (though some cars now have or offer laminated side and/or rear glass as well). It's not supposed to be a big difference but laminated windscreen glass can vary in hardness depending on what standards it has to meet. I tried to find out what standards there are, at least in Australia, but the tests were more to do with impact and abrasion resistance and how much light is scattered from the resulting wear on the glass. One dubious source (ebay) puts Laminated windscreen glass at commonly around 6-6.5 or at least above 5.5 on the MOHS scale.
http://www.ebay.com.au/gds/How-Hard-Is-a-Windshield-on-the-Mohs-Scale-/10000000205128039/g.html

I did come across an article written by a professional window cleaner who talked about using steel wool on tempered glass. There was however a warning in it regarding poorer quality tempered glass which had microscopic fabricating debris baked into the surface of the glass. He mentions this can sometime be dislodged and trapped in the steel wool or under the razor blade, if using that method, leading to microscratches. http://en.allexperts.com/q/Cleaning-2305/2009/12/Window-Cleaning-1.htm. The same could happen if your windscreen is weathered, pitted or has chips in it.

The other potential issue I guess is that less scrupulous steel wool manufacturers could be somewhat lax in making sure their product meets the correct abrasion ratings or mistakes may occur in correctly labelling/packaging steel wool products."


Personally I would never use steel wool to clean a windscreen as the risk of microscratching makes it too much of a risk for me.  Plus bits always flake off steel wool and can get caught in nooks and crannies and cause rust spots on surfaces.  A bit like the filings you get landing on your car if you park it near a rail line.  I did once try to clean my windscreen with a bathroom glass cleaner powder and a cleaning pad since it did such a great job on my bathroom glass with no scratching.  But, when cleaning my windscreen, I put too much pressure on the pad and ended up with a lot of micro scratches.  It looked fine during the day but at night when light hit the microscratches the resulting glare and "dazzle" was horrible.  I then bought a proper car windscreen glass polishing kit and wet polished the entire windscreen.  Though that took a long time, it got rid of all the scratches I'd put in and the fine wiper sweep scratches that I had been trying to get rid of in the first place.
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