A stone thrown up by another vehicle hits the windscreen and leaves a chip. Usually the damage is small, depending on the force of the impact and the size of the stone.
Repair a chip quickly: vibration, temperature swings, the demister in winter and sun in summer make cracks spread, and a small chip can soon run across the whole screen. A small chip with no legs, no bigger than a coin, can be filled with resin from a repair kit. Larger damage means replacing the screen, a job best left to a specialist workshop.
What you will need
- chip repair kit: suction-cup applicator, resin, clear curing strips
- isopropyl or ethyl alcohol, microfibre cloth
- fine pin, glass scraper blade, UV lamp if available
- WD-40 Specialist Penetrant for seized wiper arm nuts
- WD-40 Multi-Use for wiper spindle threads and pivots
Important: Never apply lubricant to the glass or bonding area: an oily film weakens resin and adhesive and causes smearing. Clean glass with alcohol only. Damage in the driver's line of sight or cracks longer than a few centimetres cannot be repaired.
Step 1
Prepare the glass
Wash the car and clean the glass around the chip with ordinary glass cleaner. Pick out loose fragments in the centre with a fine pin, then wipe the area with alcohol and dry it. Work in the shade at a moderate temperature: on hot glass resin cures too fast, on cold glass it does not flow into the cracks.
Step 2
Inject the resin
Fix the suction-cup applicator to the glass exactly over the centre of the chip. Screw the resin nozzle into the central port and feed in the resin as the kit instructions describe, so it slowly penetrates every micro-crack. Wait for the stated time, then remove the applicator.
Step 3
Finish the surface
The resin has filled the cracks, but the surface may be uneven. Add a few drops, cover with a clear curing strip and leave it in the sun, or use a UV lamp to cure it faster.
After a few minutes, peel off the strip and shave off excess resin with the blade held at a shallow angle, then clean the whole screen. Repeat if needed until even the finest leg disappears.
Step 4
Remove the wipers before replacement
If the screen has to be replaced, the wiper arms and scuttle panel come off first. The arm nuts often seize because of road salt and de-icers. Apply WD-40 Specialist Penetrant, wait a few minutes and undo them steadily so the spindle splines are not damaged.
Step 5
Cut out the old screen
A special blade is pushed through the sealant bead from inside, and a strong cutting wire is attached to its tip once it pokes out. Two people then pull the wire back and forth with handles inside and outside until the whole bead is cut; wire-pulling tools also exist. The old screen is lifted out and the frame cleaned of old adhesive.
Step 6
Bond the new screen and reassemble
A thick bead of windscreen adhesive is applied around the edge of the new glass with a gun, and two people set the screen in place with suction handles; some cars also need a trim seal. Wait the time the adhesive maker specifies before driving.
Before refitting the wipers, put a little WD-40 Multi-Use on the spindle threads and arm pivots, keeping it well away from the glass. Next time they will come off easily.
Tip: Until you can repair a chip, cover it with clear tape to keep dirt and water out of the crack.
What next
After a repair, avoid pressure washing and a hot demister for a day or two, and check the glass regularly for new chips.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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