Ideally, the best way to repair a crack in a window is to replace the damaged pane of glass. When it comes to a crack in your windshield, your best option is to have the windshield repaired or replaced professionally. Unfortunately, this may not be an option if your windshield cracks or chips in the middle of a road trip. If you can, find a mobile windshield repair company that can come to you and fix the problem or look for a windshield repair shop along your route.
When this isn't an option, there are a few stopgap measures you can take until you can secure a proper repair. These stopgaps will work for other windows too, but windshields are more likely to need immediate attention, as the motion of the vehicle when driving can cause the crack to spread faster than it would on a stationary window pane.
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Superglue and Nail Polish
Although not permanent fixes, superglue and clear nail polish can both cover a crack. This will protect the glass, creating a barrier that prevents water and other debris from getting into the crack and making it bigger. To apply these fixes, first clean the windshield and then apply the polish or glue, making sure to get some in the crack itself. Spread the polish or glue beyond the edges of the crack as well to form a seal around it.
When you do make it to a glass repair shop, make sure you tell them about your temporary fix. They will need to remove any nail polish or superglue that you applied before they can make the repair, so you need to tell them what you've done.
Taping the Crack
This repair works best on broken windows but can work on a windshield in a pinch. When you have nothing else on hand, you can apply a strip of tape over a window crack. If the crack is in your windshield or a car window, it's best to use clear shipping tape. Duct tape will do if you have nothing else, and the crack is well outside of your line of vision. You can also use duct tape on window glass in your home or office where a temporary decrease in your vision through the window isn't an issue.
A DIY Repair Kit
Auto parts stores sell DIY windshield repair kits, and you may be able to use one on your window or windshield. Use them carefully, however. Some kits deliver less-than-stellar results, and you can easily spread the crack when attaching adhesive or suction kit components to the windshield or window. Applying too much pressure to the crack can spread the crack or cause the already compromised glass to shatter.
Treat the Glass Kindly
In addition to applying temporary fixes, you must be careful with cracked glass. Avoid opening and closing cracked windows, as jostling them around can spread the crack. It's more difficult to keep a cracked windshield stationary, but you can help yourself by driving very slowly over speed bumps and not slamming the car doors or trunk. Take curves and turns a bit more gingerly than you normally would and avoid parking in direct sunlight since glass expands in the heat. If you can, avoid using the window defroster.