# Glass Polishing by Machine - Defect Correction



## Dave KG

Search the forums and you will find thousands of threads about the correction of defects from paint - hard paint, soft paint, white paint, black paint, pink paint etc etc etc... But you'll also notice a complete dearth of threads about removing defects from glass! In this thread I am going to introduce a technique and polishes I have been using on glass in testing in recent months, with further threads to come with more detailed results as well.

First things first - glass is tough! Very tough! Think the hardest paint type, and multiple by "A Large Number" and you're getting there! Many glasses you can clean with steel wool and not scratch them! So, as you can probably imagine, the use of traditional paint polishes and abrasives is rather out of the question. But how then, can we deal with marks like this on glass?




























These marks are wiper sweep marks, typical on a lot of glass. In particular, Porsche owners complain of their windscreens being afflicted by this wiper sweep marking which can be very distracting on a sunny day!

Removal of such marks requires specialist abrasives designed for task - cerium oxide is one such abrasive type, and is found in a few glass correction products. The one used for these tests is Liquid Glass, produced by Rapid Autocare. It is supplied in a kit designed for use with a drill - comes with the polish (very watery) and a couple of rayon pads. A a sponge which I am sure has a use, but not sure what it is yet - was useful for blowing my nose though.

Now, one major concern here is heat. The correction of glass requires a lot of effort owing to its hardness and this can lead to large heat build up. It is important to monitor this very carefully, as high heat can disform the glass, distorting vision and making a replacement necessary. Always be aware of this when glass polishing! Using a drill, if you stay in the same place too long, you will distort the glass - trust me 

The techniques that we have been working on for glass involve the use of a rotary polisher, a 2" backing plate (Ben at Carnaubawax Shop has them ), and 2" Rayon pads (again, Ben has these, or use the ones in the kit). The pad is primed with the watery polish, and a region of about 6" square at a time is polished as follows:


Spread at 600rpm
Begin to work at 1200rpm, light pressure and maintain this speed until polish well spread to avoid splatter later
Work at 2000rpm, light to medium pressure, slow machine movements for a minute or so
Work at 2500rpm, medium to heavy rotary pressure*, slow machine movements monitoring the heat very carefully until polish starts to dry a little
Finish at 1200 - 1500rpm, light pressure and faster machine movements

* Be very careful not to bow, or break glass with too heavy pressure and at all times monitor the heat. It should never get too hot to tap by hand, perhaps a little too hot to hold for a while.

Essentially a high-speed Zenith Point technique, with a typical set lasting three or four minutes. Care was taken throughout to monitor the heat of the glass (did I mention heat again? Its important ). The results, here on Vauxhall Astra glass, are as follows:




























A definite difference, and a definite improvement! These results have been replicated during details too thus far, most notably on a Porsche 911 which Gordon detailed recently 

Further testing is still required for dealing with deeper glass scratches, and further writeups will follow based on this and alterations of the above technique. Hopefully this will give a flavour for what is possible on glass though with a rotary polisher and the right products 

I would like to thank Gordon (caledonia) and Davy (badly_dubbed) for their help in this test  ... We three are the lab monkeys!!


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## e60mad

Quality work as always Dave. Excellent write up and already looking forward to the next installments


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## Mirror Finish Details

Interesting Dave, thanks for that.


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## chunkytfg

Any forseable issues with the laminate in the middle getting hot and possible discolouring do you think?

how about it's use on the heated front screens?


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## Dave KG

chunkytfg said:


> Any forseable issues with the laminate in the middle getting hot and possible discolouring do you think?
> 
> how about it's use on the heated front screens?


This was a heated rear screen that it was used on in this example here.

Triplex glass, the one with laminate inside as you describe, is susceptable to heat - I know this through testing  However, it was not discolouration, but rather distortion of the glass, perhaps the laminate getting too hot and distorting. But in all cases, you would want to be very carefully monitoring the heat build ups and minimising them where possible. While this glass correction is possible and can be achieved, it should be viewed IMO a bit like wet sanding - its a last resort option, required only if you cannot sleep at night because the glass scratches are so bad! The possibilities for issues are there and should always be considered, and a new pane of glass would be needed in the event of discolouring or distorting.


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## rodders

Very interesting, thanks Dave!


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## empsburna

Very interesting.


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## Rowan83

Thanks for the write up Dave, will come in very handy! :thumb:

My windscreen is a right mess


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## Planet Man

Hi Dave.

Great write up. The reason we include the Sponge is so you can clean the glass and check progress. This is needed when customers use the retail Kit as the drill is more difficult to control than a rotary and you can actually create quite a lot of mess.

Also in the instructions we advise that if any of the Glass Polish should get on the paint, to use the sponge fully wet with water to lift off the polish due to it's high level of abrasiveness:thumb:


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## ant_s

a very good read there may have to look into getting some


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## caledonia

Dave KG said:


> This was a heated rear screen that it was used on in this example here.
> 
> Triplex glass, the one with laminate inside as you describe, is susceptable to heat - I know this through testing  However, it was not discolouration, but rather distortion of the glass, perhaps the laminate getting too hot and distorting. But in all cases, you would want to be very carefully monitoring the heat build ups and minimising them where possible. While this glass correction is possible and can be achieved, it should be viewed IMO a bit like wet sanding - its a last resort option, required only if you cannot sleep at night because the glass scratches are so bad! The possibilities for issues are there and should always be considered, and a new pane of glass would be needed in the event of discolouring or distorting.


Sorry Dave you have your Glass the wrong way around.
Triplex is hardened and tempered glass. Normally side window and occasionally the rear screen.

Wind screens are standard laminated glass. The has a piece for fibre glass sandwiched in between to thin sheets of glass. Reason is safely and strength.

Triplex glass can only be hardened on two faces not the edge. So where the external flat surfaces are hardened, and can take a considerable blow. The edge if tapped will shatter into thousand of small pieces.

Both types of glass can handle heat in different ways, as our test will show. Might be a few month before we get there though. Due to up coming commitments. :lol:
Gordon.


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## robertdon777

Personally on a windscreen I wouldn't try polishing defects out. Its just not worth it - one tap with a sharp instrument and a quick phone call to Autoglass and you can have a brand new screen for 50 quid on your Insurance.

Rather that the worry of getting distorted field of vision from over zealous polishing - Not aimed at you DAVEKG - just alot of amateur polishing can ruin a screen easily.


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## Planet Man

robertdon777 said:


> Personally on a windscreen I wouldn't try polishing defects out. Its just not worth it - one tap with a sharp instrument and a quick phone call to Autoglass and you can have a brand new screen for 50 quid on your Insurance.
> 
> Rather that the worry of getting distorted field of vision from over zealous polishing - Not aimed at you DAVEKG - just alot of amateur polishing can ruin a screen easily.


Take you point on board however we have saved many screens that would have failed MOT's which would have required a new screen at substantial cost to the user.

So for around £17.00 it provides a very cost effective solution. Regarding users we have sold nearly 5000 of these kits since launch into the retail sector and have yet to have an issue with anyone damaging screens.

I would also expect the members on here to be more capable than most high street consumers so fear not regarding screen damage.

It would take some considerable time to polish the screen and warp the glass with the standard kit. We do urge caution on the instructions to be on the safe side:thumb:


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## reparebrise

caledonia said:


> Sorry Dave you have your Glass the wrong way around.
> Triplex is hardened and tempered glass. Normally side window and occasionally the rear screen.
> 
> Wind screens are standard laminated glass. The has a piece for fibre glass sandwiched in between to thin sheets of glass. Reason is safely and strength.
> 
> Triplex glass can only be hardened on two faces not the edge. So where the external flat surfaces are hardened, and can take a considerable blow. The edge if tapped will shatter into thousand of small pieces.
> 
> Both types of glass can handle heat in different ways, as our test will show. Might be a few month before we get there though. Due to up coming commitments. :lol:
> Gordon.


Gordon

A few little corrections and additions, between the layers of glass in laminated glass the product is Poly Vinyl Butyl or PVB for short. Also some new windscreens are now 5 layers, that is 3 glass and 2 PVB(to control noise), the total thickness of the assembly is not more than a regular 3 layer WS. Also a lot of high end cars use laminated glass in the side windows as well for noise control.

Effectively glass can take heat quite well, but remains very fragile(it is a liquid after all). Non hardened glass is prone to breakage when overheated. So if the WS has chips of many little inclusions, polishing it is risky. Tempered glass has a very high resistance to heat(its what is used in doors of ovens) while laminated glass is easy to deform if overheated, plus the PVB layer can cause distortion.

FYI I (and more than 30 techs) have been professionally polishing glass for the last 6 years.

Here are some examples of what can be corrected.


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## gt5500

reparebrise said:


> Gordon
> Effectively glass can take heat quite well, but remains very fragile*(it is a liquid after all). *




Being pedantic here (hey its what I do best) but glass is an amorphous solid. No I am not a scientist or a glass expert but its one of those urban legends that I read about as I like to know if they are true or not, in this case all evidence points to it being untrue. A point which was further reinforced in an episode of Qi where they said it was infact not a liquid. Anyway sorry to drift off topic but I am bored at work :lol:


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## badly_dubbed

awesome day as always, and glad i could learn something again :thumb:

Cheers

Davy


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## Bigpikle

reparebrise said:


> Gordon
> 
> A few little corrections and additions, between the layers of glass in laminated glass the product is Poly Vinyl Butyl or PVB for short. Also some new windscreens are now 5 layers, that is 3 glass and 2 PVB(to control noise), the total thickness of the assembly is not more than a regular 3 layer WS. Also a lot of high end cars use laminated glass in the side windows as well for noise control.
> 
> Effectively glass can take heat quite well, but remains very fragile(it is a liquid after all). Non hardened glass is prone to breakage when overheated. So if the WS has chips of many little inclusions, polishing it is risky. Tempered glass has a very high resistance to heat(its what is used in doors of ovens) while laminated glass is easy to deform if overheated, plus the PVB layer can cause distortion.
> 
> FYI I (and more than 30 techs) have been professionally polishing glass for the last 6 years.


interesting thread Dave (& Gordon) and good post Yvan.

Having worked for Pilkingtons for a few years advising on glass specs and replacement, in applications such as fire, safety and security (how to keep bad guys with sledgehammers out of jewellers window displays ) I have a bit of knowledge on things glass and glazing...

One thing to remember about laminated glass (in screens mostly) is that due to the fact it is multi-layered, each glass layer is very thin (compared to tempered glass of the same total thickness). While these layers are laminated together (for various safety and noise reasons), each only has the strength and heat resistance it would have as an individual piece of glass. Many people think laminated glass is somehow stronger than a single layer of glass, but in actual fact it is weaker and easier to break than a single piece of glass of the same thickness. In other words 2 pieces of 2mm glass laminated together is easier to break than a piece of 4mm glass. The reason its used in screens is that impacts crack the glass but the PVB layer keeps it intact and still serviceable as a WS. Its a legal requirement to use it in many domestic and commercial applications for that reason, as it avoids those lethal jagged shards popular in all the horror films, and the PVB cuts out pretty much all UV as well, making it ideal for shop windows etc.

Heat treating (toughening or tempered) glass changes the structure of the glass by heating and rapid cooling. It effectively creates a piece of glass under tension - the inner molecules want to expand and the outer layer are keeping it together. This makes it strong and heat resistant as Yvan says, and it resists big impacts with blunt objects very well, BUT pierce that outer layer and the entire piece fractures into thousands of small pieces. You could hit your rear screen with a big hammer and probably not break it but lean on it with a stiletto heel and its toast! The key thing is each of the broken pieces has no real sharp edges that could cut you to ribbons, so its safe.

Look forward to more glass polishing tests and reports from you and having a play with this myself at some point :thumb:


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## Brian.

robertdon777 said:


> Personally on a windscreen I wouldn't try polishing defects out. Its just not worth it - one tap with a sharp instrument and a quick phone call to Autoglass and you can have a brand new screen for 50 quid on your Insurance.


In my case it is a great thing being able to polish glass. For example, I want to keep my 309 Goodwood as original as possible so keeping the original glass (which every bit of glass is at the moment) is great in my eyes.


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## mkv

Having worked for 2 of the major aoutmotive glass replacement companys in my time, i find this interesting.
Toughend glass is so fragile. As Bigpikle says, it basically tensioned glass. Ive had heated rears blow up with the slightest touch to them. Also seen them dropped by accident and stay complete. Little or no heat, in my opinion is the only way to go with polishing toughend glass.
Laminated glass is different story. All modern front screens and some rear heated screens are in laminate. Some older vehicles may still have toughend. More heat can be usd to polish, but be carefull as too much heat in 1 area can cause delamination. Basically where the the plastic laminate layer betwen the 2 layers of glass starts to bubble.
As with all industries, glass technology has come a long way and polishing by Dave is looking good.
Something you should consider is the quality of the glass. That may sound silly, but not all glass is the same. A certain replacment company, in orange vans, have there own manufacturing facilities. To be honest some of the screens that came out of there was shocking, warped screens were a common occurance. The screens just felt of an inferior quality.
The other major company in white vans on the other hand have much better pedigree. There group company actually owns or has part shares in most of the glass manufacture. So the glass they fit is mainly OEM. At least you know, most of the time, you are getting factory standard replacement.

Steve


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## daffyduck

How much is the kit?

You say this is a last resort, what other measures could be taken to remove wiper scratches?

Does it have to be a rotary, would a g220 be no use?


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## fonzi83

Very good and useful topic. Congratulations to the author :thumb:


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## fonzi83

Dave,

I have one question for you ... !

What is your opinion about this product CarPlan ... Can you use mashine polish on the glass ?

http://www.carplan.co.uk/index.cfm?product=66


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## Planet Man

fonzi83 said:


> Dave,
> 
> I have one question for you ... !
> 
> What is your opinion about this product CarPlan ... Can you use mashine polish on the glass ?
> 
> http://www.carplan.co.uk/index.cfm?product=66


It is a good glass cleaner but nowhere near as effective as our Glass Polish Kit for removing scratches.:thumb:

If they were to get enough glass polish in that bottle it would cost around £25-£30 per bottle.


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## 123quackers

Interesting read thanks guys :thumb:..Looking forward to more updates on this..

Mercedes S class has double glazing would this be different again?? As normally they are gas filled..


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## octane

Excellent stuff Dave, thanks for taking the time and effort yet again to educate us mere mortals.

Out of curiosity and im probally being stupid but do you think using the raw materials that make glass would be of any use for revitalising it. Limestone, Sand, Soda ash and whatever else is used.
Not exactly off the shelf stuff....lol


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## josh263uk

It mentions in the 2st post about using a rotary, what speeds would you use with a DA?


Thanks

Josh


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## wyliss

I'm about to attempt this. Has there been any advancement in polishes etc?
My 911 windscreen is covered in wiper blade marks were the previous owner negated to purchase new ones!!
Cheers


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## justinio

Cerium oxide, Rayon pad, a DA or rotary and a whole lot of time. That's about all you'll need.


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## wyliss

Is 'Ceriglass' any good?


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