You car windshield is not just “another piece of glass.” It is one of the most important vehicle safety features and the only thing between you and everything else on the road. Your windshield is comprised of 3 laminated layers. The top layer on the outside is typically UV glass as is the bottom layer, inside the car. The middle layer is soft and comprised of PVB (Polyvinyl butyral (or PVB) a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility.
Trained windshield repair technicians inject an acrylic resin into the damaged area on your windshield using special tools and high pressure. The process involves both pressure and vacuum cycles. This is where do-it-yourself kits fall short and usually fail. Do-it yourself kits only allow for one pressure cycle. In other words, you get one shot. If it fills, you are lucky and what ever the result, you are stuck with it because once the resin is injected and cured (hardened) there is no do-over. The do-it-yourself kit does not allow for a vacuum cycle so there is not way to remove trapped dirt, debris and moisture from the damaged area. The results are usually cloudy at best and unpredictable. In short, you risk ruining your windshield to save money and in the long run, home kits lead to the need for a costly windshield replacement.
While on the topic, I might also mention that attempting to repair a damaged windshield with household products, such as, Crazy Glue, and nail polish will ensure the need for a replacement windshield.
There is no substitute for using a trained professional when it comes to something as significant as one of your vehicle’s most important safety features.