Tech Tip: Properly Curing Long Cracks

Recently, I repaired a long crack, but when I removed my curing light, there were three spots where the resin did not cure. Am I curing long cracks correctly? What could possibly cause that?

Below, I detailed the type of damage I was repairing and the process I used to repair it. Spoiler alert: You can skip to the last few paragraphs of this article for an answer to the above question and a few additional tips for repairing long cracks.

Last weekend I had to repair a 12” crack that started under the molding at the edge of the glass. There was no visible impact point that I could detect, so I assumed it was a stress crack. Some technicians don’t repair edge cracks or stress cracks, but I’ve had pretty good luck repairing them over the years, so I don’t shy away from them.

Terminate the End of the Crack

The first step I took was locate the end of the crack and drilled a hole using a tapered carbide bur. I used my Delta Kits spring hammer to double-check the depth of the hole and pop a mini-bullseye about 1 mm past the end of the crack. As expected, the end of the crack ran into the hole, so I knew it was properly terminated.

Next I drilled another hole about 3 mm in from the molding at the edge of the glass and popped another bullseye. This second bullseye was necessary because I was unable to remove the molding and could not get to the edge of the glass. Adding an anchor here greatly reduces the chance of the crack opening back up, especially a stress crack.

Temperature of the Glass and Resin

The temperature in my shop was approximately 60 °F, but it was only about 30 °F outside, so the windshield was pretty cold. I used a hair dryer to direct warm air over the damaged area for about 10 minutes until the temperature of the glass was about 80 °F. I also taped a bottle of our Premium Bond 20 resin to the glass a few inches away from the crack, so it would warm to the same temperature as the glass. Note: Once the glass was warmed to the proper working temperature, I redirected the heat a few inches below the crack to maintain the glass temperature and left the hair dryer on throughout the repair process.

Remove moisture from the Break

The driver told me the crack had been in the windshield for less than a week, but it had been raining, so the biggest challenge was to remove the water before filling the damage with resin. I use a moisture evaporator to remove water from bulls eyes, star breaks, combinations and short cracks, but I opted to use a heat gun in this instance due to the length of the crack and the time I had to complete the repair. When using a heat gun, you must be careful not to overheat the glass and cause delamination. I have a gun with variable heat settings and a restricted nozzle, so I can control the heat very well.

During the moisture removal process, I could see the crack closing up some as the glass expanded. Once I was sure I had all the water out I used my Delta Kits heat exchanger to cool the glass on both sides of the crack back to about 80 °F and watched the crack open up again.

Injecting resin into the Crack

I mounted my Delta Kits B250 bridge over the drilled hole near the molding and initiated the injection cycle. For the first 6” the warm resin flowed into the crack quickly but then slowed to a crawl, so I started adding more resin to the surface of the crack about 1 mm behind where the resin had stopped flowing and watched as the resin quickly filled another inch or so. Continuing this process until the crack was filled to the end.

To remove a couple of small bubbles that were trapped inside one of the drill holes, I inserted a straight pin into the hole and ran a little more resin down the pin, into the hole, as I pulled the pin out.

Next I removed the bridge, ran a small bead of resin the length of the crack and placed a drop of Premium Bond 3000 over each drill hole. Since the crack was straight, I used two curing strips, instead of curing film, to cover the filled crack.

Curing

I plugged my long crack UV curing lamp into my portable 12V battery.  I cured the crack for a full 10 minutes, twice as long as I cure any other type of damage. It was probably overkill, but I don’t take any chances with stress cracks.

When I removed the curing strips, I found there were three sections that did not cure. Two of these sections were about 12 mm long and the other was about 3 mm long. The two 12 mm sections were positioned outside the curing area of the long crack light; the 3 mm long section was located where the two curing strips had overlapped, both rookie mistakes!

Three curing tabs, and a few more drops of resin later I cured the crack for another 10 minutes. Once cured, I removed the tabs, scraped the excess resin off the surface, and polished the pit resin in the drill holes. The repaired crack looked great, and the customer was happy.

So, What Should I have Done to Avoid the Three Uncured Spots?

1. Move the light 1” left or right halfway through the curing process. Note: if you are not using a Delta Kits’ long crack UV lamp you may need to move the light more than 1”, more than once, and will need to allow for extended cure times to make sure the entire crack is properly cured.
2. Use one long strip of curing film rather than two curing strips, or butt the curing strips together. Then use a curing tab and an extra drop of resin to cover the joint before curing.

Why Did I feel it was Important to Write this Tech Tip?

1. Technicians are often afraid of long crack repair because they experience odd problems like inconsistent curing. They do not encounter  inconsistent curing when repairing other types of damage.
2. This is a reminder to old timers like me. No matter how confident you are in your abilities and your equipment. Make sure you don’t overlook little things that can result in big problems. Not curing a crack properly has a very high failure rate.
3. For newbies who may not know what stress cracks are, or why they should be anchored may run into difficulties. For example, the difficulties you encounter when repairing long cracks vs other types of damage.
4. Many technicians don’t have long crack lights. So, they try to move their small lights multiple times. Often get in a hurry and miss spots or do not allow for proper cure times.
5. Reminder to carefully remove moisture and return glass to proper working temperature before repairing long cracks.
6. Reminder to maintain consistent glass and resin temperature prior to and throughout the repair process whenever possible.