Did you ever wonder why we are instructed to “leave 1-2 mm of existing bead” when replacing a windshield? I did, so over the years, whenever I talked to an adhesive manufacturer’s representative, I asked the purpose for that recommendation. Most said it was to prevent damage to the body of the vehicle. However, there are a few other benefits to leaving a little existing urethane attached.
We must use a cutting tool to remove most of the existing urethane. Damage prevention is a legitimate reason to leave a measured amount of urethane in place. Whether you use a utility bladed tool, a scraper or one of many specialized tools invented for the sole purpose of cutting back the existing bead, you run a risk of scratching or gouging the paint. If you take the time to remove most of the urethane and then concentrate on leaving a portion of that urethane intact (the 1-2 mm), the result will be less damage to the painted pinchweld and less premature corrosion.
However, I believe that there are a couple of other reasons for leaving a little of the existing adhesive. The most important reason is that, by leaving a portion of the existing bead, it makes it possible to replace the glass a second or third time and still have the ability to expose a portion of the OE bead.
The OE bead is something that we know. We know the adhesive meets the Federal Motor Vehicle Safety Standards because the automaker attests to that fact based on crash tests and results. We know that it was applied properly because during the first installation we stripped the existing bead in a way that proves that it is adhered by pulling the bead while trimming it back.
A bead applied by another replacement technician is something we don’t know. Did the previous technician clean the body properly? Did he use an adhesive that meets the OE specifications? Did he prep the pinchweld bead to accept the fresh adhesive? Did he apply it properly to meet the OE requirements for safety? Did he allow the adhesive to cure properly before release? We don’t know. So, if we can get back to the OE bead by trimming it back under the previously applied adhesive, we will be better off.
The other reason for leaving the recommended 1-2 mm of existing OE bead is the adhesion benefit. Any adhesive sticks better to itself than to another surface. This is due to molecular entanglement bonding. If you leave a portion of the existing bead and you apply another fresh bead on the newly exposed existing bead, your bond cannot be better. The reason is that 100 percent of the molecules in bead surface will entangle with 100 percent of the molecules in the other bead, thus making it the strongest bond possible. Leaving a portion of the existing bead assures that there is enough existing bead to bond to the new adhesive.
The final reason is cost savings. When I owned my automotive glass replacement shop, I always looked for ways to save money and add more profit. If I can strip the old bead without damaging the pinchweld paint, I don’t have to prime the body. If I don’t prime the body, I save the cost of having to buy large amounts of body primer. This saves me money and improves my bottom line.
So there you have it. These are the many reasons why a technician should leave 1-2 mm of existing urethane when replacing the stationary glass parts of a vehicle. Will this take more time and effort? No. Invest some time in training and practicing. Buy the tools to do it right, and make sure your tools are sharp and ready to go. By doing this, the time and effort is the same as stripping it clean. Plus, the technician gets the peace of mind that the job was done right.
