Is the crack volatile? How long has it been there? Is it moving?
If there is moisture in the crack it has to be removed, and the best way to do that is with heat, so although I prefer not to submit long cracks to drastic temperature changes I do heat cracks to remove moisture. The important thing to remember here is that you want to warm the glass slowly, and if at all possible warm the entire windshield. Once the glass is warm, a heat gun with a concentrated tip or a torch may be used to dry out the break. CAUTION: This is not for the faint of heart. As most of you know I am not a big fan of using a torch or heat gun on damaged glass, but when it comes to long cracks moisture evaporators just don’t always cut it. However, using a torch or heat gun greatly increases the chance of a crack out, so I strongly suggest that you only use those methods if you have practiced this technique at least a couple dozen times.
If the crack moves when you apply light pressure near the end or if the customer tells you it has moved within the last few days, I recommend drilling the end and popping a small bullseye at the end with a Delta Kits Slide Hammer. I also recommend drilling any crack over 3″ long as a precautionary measure.
If the windshield is 70 degrees F or above and there is no water in the crack I would not heat the glass. In fact, if the crack does not stem from a star or combination break that will also need to be filled I would not warm the glass unless it was under 60 degrees F. The reason for this is that as the glass is warmed it expands and if warmed too much the crack may expand and make injecting resin more difficult.
If you do have to warm the glass, do so slowly as stated above. Defrosters work well if you can run the vehicle during the repair process, but you should also warm the outside of the glass whenever possible. Generally I like using a hair dryer for warming a break, but when dealing with long cracks I prefer using heat lamps or an electric blanket draped across the glass. This allows you to warm the glass slowly and keep it warm during the repair process. However I must admit that I don’t typically carry an electric blanket or heat lamps with me, so in most cases I end up positioning my hair dryer so it blows warm (not hot) air across the crack during the repair process. Warm the glass prior to starting the repair and do not apply spot heat to a long crack during the pressure cycle.
I don’t typically use crack expanders unless I expect to have a troublesome spot in a crack, but if you have drilled the end of the crack, popped a bullseye to terminate the crack, and have warmed the glass to the proper temperature, you may want to place a crack expander every 6″ or so and carefully apply light pressure. If the crack has partially closed due to the drying or warming process, the crack expander will open it back up and make filling with resin easier. Be prepared to remove crack expanders quickly if the crack moves past the termination point. Otherwise leave them on until the crack is completely filled with resin and then remove them just prior to curing.
Important things to remember when filling long cracks regardless of weather conditions.
1) Be sure you identify the “true” end of the crack and mark it from the inside of the glass with a Sharpie prior to drilling and creating a termination point. Remove the sharpie mark after the repair is complete.
2) Be sure there is no moisture in the crack.
3) Be sure you position yourself at such an angle that you can see the resin filling the crack. Technicians often mistake air for light reflection or vice versa, but if you watch the resin flow through the crack you will know when it is properly filled.
4) Be sure the resin flows all the way to the end of the crack.
5) Never attempt a long crack repair unless you are comfortable doing so. Long crack repair takes practice, and practice should never be performed on a customer’s vehicle. Use practice glass.
6) Allow more time for crack repairs, especially in cold weather.
7) Be sure to match the resin temperature to the glass temperature. It does not have to be exact, but keeping it within 10 degrees F will minimize the chance of a crack out and help fill cracks faster.
In summary, crack repairs in cold weather can be challenging, but with enough practice, the proper tools, and the right technique, they can be accomplished successfully.