One of the most difficult skills a new technician needs to master is how to trim back the existing urethane to the proper point. The typical advice, used in the industry for decades, is to trim the existing bead to 1-2 mm or a 1/16 of an inch. Though we have all heard this phrase used frequently, most people don’t trim back the existing bead and then measure the thickness of the urethane left behind to make sure that it was exactly a 1/16th of an inch. If the bead is a 1/32 or 3/16 of an inch, is that wrong? No it is not.
The purpose of leaving some existing urethane is twofold. The most important reason is to discourage the technician from going too close to the painted metal and thus damaging the pinchweld and causing corrosion. The second reason is to remove most of the aged and possibly deteriorated urethane and to leave behind a perfect virgin bonding surface for the new urethane to adhere to. Both of these goals are important to the success of the bond and the protection of the vehicle, but the exact amount of the existing urethane to leave behind is not as important as the end result.
The bottom line is don’t damage the pinchweld, and be sure to remove most of the old bead. I frequently see new technicians struggle with this directive, prolonging the installation process or causing the quality of the installation to suffer.
In the next few of posts I will address the proper procedures for full strip-outs and the ramifications when the strip-out is done incorrectly. Today, I will address the tools available for this purpose.
There is a wide array of tools to perform the act of stripping the old urethane from the body. Some of these tools are commonly found in a hardware store for affordable prices. Others bring highly sophisticated designs that make the task easy and reduce the chance of damage. However, with practice and care, they all work fine. I have yet to stop a technician from using a tool they have success with. Yes, I do have my preferences, which usually are formed by the length of the learning curve but there are arguments for and against for every tool I’ve seen. Here is just a small list with my comments:
Razor Blade Scraper: These are inexpensive. It is a single-edged razor blade that can be changed often for little cost. Once learned, it is easy to use. The blade can be too wide for the average pinchweld. The tool can strip too close to the body and the blade is sharp and can easily cause damage until mastered.
Utility knife/Long-handled Utility, Olfa® Knife: These are relatively inexpensive. You must use the proper knife blades or the blades can snap. Some blades can be a little more expensive. It takes more practice to learn and master this method. The tool can damage headliners and interior garnish mouldings. It can also strip too close to the body. The blade is sharp and can easily cause damage until mastered.
Scrapers, Chisels, Putty Knives: These are relatively inexpensive. The blades need to be sharpened and kept honed for productive use. If used incorrectly, the blade can cause paint damage. It is a little harder to use than replaceable blade-type tools.
Replaceable Blade Scrapers: These are sometimes called “inline” utility knifes. These use the action of a scraper and the replaceable blades like a utility knife. They are medium-priced. The blades can be expensive in comparison to utility blades but they can be sharpened and reused. Multiple blade widths allow for use on any sized pinchweld.
Pinchweld Prep Tools: These tools are specifically designed for automotive glass installation and they vary in style and use. They can be expensive, but they have protections built in to reduce damage to the vehicle and usually provide a short learning curve.
There are technicians getting excellent results with all of these tools. It is worthwhile to take the time to figure out what works best for you. Remember, the key is to get a smooth, damage-free removal of the existing bead, leaving the virgin surface for the proper bonding of your new urethane.
