The first step in repairing a windshield is cleaning the damaged area. For a successful repair job it is important that any contaminants (dirt, moisture, etc.) are removed from the damaged site. This means that it is important to get chips, dings and cracks repaired as soon as possible. Moisture can expand and contract with temperature changes, causing cracks and starbursts to grow. Dirt or other contaminants can create problems in filling the damaged areas and leave unsightly scars after the work is done.
Second, the windshield must be repaired by filling the damaged glass (via drilling and injection) with a resin that mimics glass. The resin is injected into the drilled area after a vacuum is created. Creating the vacuum prevents lingering air pockets and reduces the stress on the glass as resin is infused. The resin has an index of refraction that nearly matches that of glass; therefore, when light shines through the glass, the repaired area almost disappears. Most repairs do leave a small 'scar', however, and major cracks or damage that interfere with the driver's sight should be discussed with a professional. It may be necessary to replace the windshield in these cases.
Finally, the resin must be 'cured' or set, so that it is no longer a liquid, but a solid adhesive that binds the broken glass together. The curing process is accelerated through the use of a specialized light so that the car can be driven immediately after the repair is done.