Preparation of the Pinchweld
Glass preparation is very important to the success of the bonding chain, but proper preparation of the body’s frame (the pinchweld) is equally important. In most cases we don’t actually prep the pinchweld, but we prep the bonding surface applied to the pinchweld. The bonding surface is most often the existing original bead of urethane applied by the vehicle manufacturer, but it could be the OE paint, OE primer or aftermarket primer. In any case, the bonding must be attained by following the adhesive manufacturer’s instructions exactly as written.
The first universal understanding of bonding adhesives is that an adhesive will always adhere better to itself, or its chemical twin, than to any other surface. Thus, urethane adheres better to urethane than it does to other surfaces or coatings. That being said, it is our goal to reach a pure uncontaminated urethane surface to which we will bond. Getting there is the skilled part of the installation.
Our goal should be to cut out the glass from the vehicle and strip the urethane from the body cleanly without scratching any of the painted surfaces. And, in a perfect world, all technicians would cleanly re-install the glass without any excessive urethane oozing to the pinchweld wall leaving 1-2 mm (millimeter) of original urethane by which the next technician could replicate the first installation and so on and so on. However, that is not how things are in the real world. Many times the technician finds improper prior installations, sloppy body work or corrosion that has to be treated. This means that pinchweld preparation is more difficult to attain and experienced decision-making must be implemented to correct the problem or create the proper bond.
The next series of posts I plan to share is how we deal with each of the situations that might present itself in preparation of the bonding surface. All of these discussions are in the adhesive instructions of your chosen urethane product, but I have found that the instructions are either forgotten or seldom taught because they are not followed or understood universally.
Basic Body Preparation
The basic body preparation instruction is the first one that every urethane manufacturer prints in their instructions and teaches. The reason is because it is the scenario that is most commonly seen by the technician; it is the simplest to explain, easiest in theory and most cost effective.
Remove the glass, trim back the existing urethane to 1-2 mm (1/16 of an inch), apply urethane and set the glass.
There is no need to add primer to the existing bead because urethane sticks best too itself.
If there are no scratches caused, no primer is needed and cost can be reduced.
This is the perfect world scenario that unfortunately many of us see only seldom due to the issues named above. However, we should keep trying to attain this perfect bonding scenario because it is the simplest, easiest and least expensive way to go. We can increase the frequency of this basic preparation with the use of technologically advanced tools and the implementation of improved technique. Eliminate scratches and contamination and you have contributed greatly to the steps necessary to the perfect installation.
What do I mean by technologically advanced tools and improved technique? I mean the new wire-out tools, power tools and how they’re used, and special cold knife blades all contribute to less body damage. Strip out tools and the techniques in stripping the urethane from the body also reduce the chance of paint damage. Less body damage means less additional preparation is needed for bonding and more profits are realized due to increased productivity.
Next week we will look at the more challenging body preparation issues.