Saturday, 5 August 2017

Polishing Glass

Glass Technology

[: Modern windshield or windscreens are generally made of laminated safety glass, a type of treated glass, which consists of two (typically) curved sheets of glass with a plastic layer laminated between them for safety, and are glued into the window frame.]

The term windshield is used generally throughout North America. The term windscreen is the usual term in the UK and Australia/New Zealand

Glass windshields first appeared around 1905.  The first automobile windshields were a luxury, sold as optional equipment to motorists who needed something a little more dashing than goggles. With the invention of safety glass, tempered glass (tempering is a heat treatment) to make it especially hard and resistant to shattering. This type of windshield was popular well into the middle of the century, but it was eventually replaced by windshields made of laminated glass

Automakers began using laminated safety glass, also known as auto glass, for automobile windshields in 1927. To make laminated safety glass, the manufacturer sandwiches a thin layer of a flexible clear plastic film called polyvinyl butyral (PVB) between two or more pieces of glass. The plastic film holds the glass in place when the glass breaks, helping to lessen injuries from flying glass.

PPG Industries has developed Optech clear coating, it’s made up of a single component designed to provide improved resistance to marring and environmental etch.

According to PPG, the high solids formulation, developed over a six year period, allows it to bond with an adhesive to join the windshield, backlight and sidelight windows to vehicle bodies thereby eliminating the need for a urethane primer commonly used to ensure adhesion.

Optech also meets the Federal Motor Vehicle Safety Standard for windshield retention during rollover accidents and crashes. The technology can improve scratch and etch resistance characteristics by approximately 25 to 100 percent when compared to conventional one-component clear coats that are currently used, says PPG. Optech is currently being used in two plants in North America.

Laminated glass is a multilayer unit consisting of a plastic layer surrounded by two sheets of glass. In many countries, including the U.S., auto windshields are required by law to be made of laminated glass. Laminated glass can bend slightly under impact and is less likely to shatter than normal safety glass. This quality reduces the risk of injury to the automobile's passengers.

After the glass is tempered and cleaned, it goes through a laminating process. In this process, two sheets of glass are bonded together with a layer of plastic (the plastic layer goes inside the two glass sheets). The lamination takes place in an autoclave, a special oven that uses both heat and pressure to form a single, strong unit that is resistant to tearing. The plastic interlayer is often tinted to act as an ultraviolet filter.

When laminated glass is broken, the broken pieces of glass remain bound to the internal tear-resistant plastic layer, and the broken sheet remains transparent. Thus, visibility remains good. Unlike traditional safety glass, laminated glass can be further processed—cut, drilled, and edge-worked, as necessary. A typical laminated windshield is very thin: each glass layer is approximately 0.03 inch (0.75 millimeters) thick, while the plastic interlayer is approximately 0.098 inches (2.5 millimeters) thick.

Types of Glass Surface Corrosion
 a) Stage I Corrosion [: defined as a surface with light to moderate corrosion damage to the paint surface] 

 b) Stage II Corrosion [: definition when the dirt/corrosion deposits are no longer on the surface but have started to break down the molecular structure, leaving an etched or white haze on the surface after the stain has been removed, with moderate to serious stains/damage] (See also Glass Cleaning Process article)

Rock Chips
These microchips are caused by road grit impacting the glass surface and leaving small holes that show up as white spots when waxing or applying a sealant. In a recent windshield damage study conducted by the Motor Industry Research Association (MIRA), it was found that; extreme temperatures and structural fatigue will cause existing windshield chips to spread quickly, requiring glass replacement.

Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts, this differential expansion can be understood in terms of stress or of strain, equivalently, at some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail. This occurs if you wash a cold surface with warm/hot water or conversely a warm/hot surface with cold water Thermal shock can be prevented by reducing the thermal gradient seen by the object ( i.e. by changing its temperature more slowly)

In cold weather conditions, windshields will develop a more concave shape. As a result, windshield chips and dings are more likely to spread in a horizontal fashion. Chips were likely to spread more than 80 percent of the time at temperatures of 14.F degrees (-10.C). Those tested at 32.F (0.C) were nearly 60 percent likely to crack.

Warm weather conditions cause windshields chips to spread more vertically. The same study conducted suggested that the heat itself distorts the shape of the windshield glass, which can lead to cracking by itself. However, it was also found that the cooling effect of the air conditioning unit is likely to set off a crack.

Driving over a speed bump, pothole or other uneven terrains can cause a chip to become a fracture. The force radiates through the car and puts additional pressure on the edges of the chip, which can quickly lead to a glass fracture.

Although it is not possible to suggest how long it will take for a chip to spread, the study suggests that stressors like weather changes and simple driving will eventually cause or lengthen a windshield crack.
 What kind of damages can be repaired?
A windshield repair is a process that uses skill to repair stone damage on a windshield instead of replacing it altogether. Stone damage or stone "chips" up to the size of a quarter can usually be polished out, however, if you can catch your fingernail in the scratch 0.4 Mil (10µ) deep, you should consult a professional auto glass company

Best results are obtained when the damage is repaired right away before contamination occurs from car washing, road grime, or inclement weather.  

 Once the glass surface has been washed and cleaned (detailers clay) any minor imperfections can be removed with an abrasive glass polish (CarPro CeriGlass, Autoglym Car Class Polish or Zaino Z-12 Clear-View Glass Polish) These products remove "light" scratches, acid water spots, bird and bug stains, including alkaline mineral deposits, and then smoothes out etched glass

Should I repair or replace my windshield?
Be aware some imperfections cannot be removed (by doing so you will compromise the glass surface optics) and you may have to replace it. If you can catch your fingernail in the scratch you should consult a professional glass repair/replacement company.

A replacement windscreen can sometimes be claimed on an insurance policy without loss of no-claim bonus, dependent upon coverage (check first with the insurance company).Some windscreen ‘glass’ is actually a plastic resin and will scratch easily, consult your dealer to find out what material was used for the OEM windshield.

There are various types of windshield glass; some newer cars have acoustic glass or rain sensors built in, so be sure you get the correct type of glass. Aftermarket glass is not required to meet the same Federal Motor Vehicle Safety Standards (FMVSS) regulations as OEM glass

An OEM glass is usually stronger and this may be important when you realize that the windshield becomes the backing plate for your air bags in a crash. The glass windshield provides up to 60% of the cage strength in the event of a rollover accident. The OEM glass is more expensive as it is more costly to make. Yet the windshield may save your life or your family's lives in a crash.  See also -

Glass Polishing Pad
Griot's Garage synthetic 6-inch glass polishing pads for use with a random orbital polisher Use these pads along with a glass polish to remove etched water marks and other tenacious surface residues. Once thoroughly cleaned they can be re- used

Alternative - Lake County - GPS PRO Glass Polishing System

Glass Polishing

Auto glass frequently gets scratched, chipped or cracked due to road debris and normal wear and tear; scratched auto glass can be polished with zero distortion using specific abrasives (cerium oxide). Common sense dictates that if you want to remove a scratch, then you must remove some of the material (paint, glass, etc) to get down to the bottom of the scratch.

Modern automotive windshields hardness is 6/10 on the Moh’s harness scale and contains a thin plastic sheet between two layers of glass (interior and exterior). Glass is very hard and t he polishing process for the correction of scratches requires pressure, which combined with kinetic energy produces friction heat, if the glass surface becomes too hot the plastic sheeting can warp leading to distortion. When polishing glass use capacious amounts of water and periodically check the surface temperature

Use caution when polishing glass, especially with polishes that contain abrasives like cerium oxide (belonging to the group of elements known as rare earth) use optical grade only on auto glass and observe all necessary safety precautions (see also personal protection equipment (PPE) with regard to the dust produced. They also have the potential to damage glass by removing layers; compromising its optical clarity, strength and rigidity, necessitating replacement An airbag relies on the glass windshield as it ‘backing plate’ (i.e. it supports the inflated bag and gives it the necessary rigidity, as it is unable to arrest forward motion without some kind of strong support) so don’t do anything that would compromise it.

Once the glass surface has been washed and cleaned (detailers clay) any minor imperfections can be removed with an abrasive glass polish (Autoglym Car Class Polish or Zaino Z-12 Clear-View Glass Polish) Removes "light" scratches, acid water spots, bird and bug stains, including alkaline mineral deposits, and then smoothes out etched glass

If you can catch your fingernail in the scratch 0.4 Mil (10µ) deep, you should consult a professional glass repair/replacement company.

• Clean glass to remove dirt, dust or road grime from surfaces  
• Use masking tape to protect windscreen rubber sealing
• Apply clay (Magic Clay®) and lubricant solution (Woolite™ / Water 5:1) onto the glass surface (Do not apply to plastic as dulling may occur.)
• Apply a quarter (coin) size amounts of CarPro CeriGlass and an LC Glass polishing pad, is made of dense polyurethane foam with very small cells. The pad’s density enables it to keep the glass polish on the surface, rather than soaking into the foam. You’ll use less polish to get the job done, despite its density; the pad still has plenty of flexibility to conform to the curvature of the glass, speed #4 on a PC random orbital polisher
• Heat caused by abrasive polishing can soften and damage glass; foam pad and product used must minimize heat build-up. Since glass is a poor heat conductor any friction heat built up from polishing must be kept to a minimum by using sufficient product and regular misting of both the polishing pad and glass surface with water.
• Apply to half of the windshield, polishing first in an up and down motion then in a left-to-right motion and then repeat on the other side, proceed to other glass surfaces.
• Go over the glass several times in each direction, the glass will polish virtually residue free.
• Wipe off any residue, and polish with a clean dry 100% cotton Micro fibre cloth.
• Inspect glass for clarity and smoothness. Repeat if necessary.

Alternative products/methods:
a) Apply Zaino Z-12 with a damp cotton cloth to cool glass, apply to small sections at a time and allow to completely dry, buffing with a clean dry cotton towel. If there is any smearing or residue left just use Stoner’s Invisible Glass or any good quality glass cleaner to remove excess residue.
b) Glass Technology Inc -  

Alternative pads- these 4-inch ‘Metal Polishing Pads’ are made from 600 grit nylon (Scotchbrite®) and are designed for hard/rough surfaces like the aluminum diamond plate, pitted aluminum, chrome, and glass. TOL

Glass Scratch / Rock Chip Removers
This glass polishing kit comprises one 2-inch diameter felt bob mounted on a ¼-inch shaft, with a tub of abrasive optical grade Cerium powder. Cerium (IV) oxide is a pale yellow-white powder with the chemical formula CeO2. It is insoluble in water and moderately soluble in strong mineral acids. CeO2 is slightly hygroscopic and will absorb small amounts of moisture and CO2 from the atmosphere.

This will remove glass scratches that you can’t feel with your fingernail (approximately 0.004") To remove scratches that you cannot feel with your fingernail, mark them on the inside of the glass with a crayon or a piece of tape. (Deeper scratches may not be possible to completely remove.)

Tape off glass surrounds with 3M Green Tape masking tape, as it’s water-resistant whereas the 3M Blue is not. Mix up a small quantity of the abrasive powder with some water to make wet slurry, and dip the flat end of the felt bob in it and use the fastest electric drill speed available, work the slurry up and down across the scratch until it is removed, keep the surface wet - do not allow the slurry to dry out, otherwise the glass will get hot -  

Note -  Do not use an abrasive polish on Targa ‘glass’ tops as they are actually a plastic (Lexan) and an abrasive will remove the tint Suggested product- Plexus (BTI Chemicals Plexus® Glass Polish & Cleaner)

Glass Polishing Kit for Deep Scratches most glass polishes contain Cerium Oxide an optical polishing compound (belonging to the group of elements known as rare earth) all necessary precautions should be used with regard to the dust produced. Removes deep scratches from automotive glass and save the replacement costs, this abrasive system literally grinds away imperfections until scratches disappear, works best with a Rotary set at 1500-2000 RPM.

The Lake Country Glass Restoration Kit #2 is for heavy duty glass polishing. This kit will polish and restore automotive glass and other glass surfaces. This machine powered system restores clarity by removing wiper tracks, light scratches, water spots, and other minor defects. Designed for use on automotive windshields and Automotive Glass; most other glass surfaces. For use with rotary buffers or dual-action polishers, the kit contains enough materials to polish and seal approximately 7,200 square inches.

Warning- these systems remove glass, maintain a wet surface and keep pad moving to avoid heat build-up and optical distortion

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