Partial Cut Defined
A man called me this week with a very interesting question so I thought I would share my thoughts. He asked if the result realized from a wire cut out on the urethane bead met the definition of a full strip. He said that once the glass was removed with a wire, the existing bead reaches the point where a trim back is not necessary. This would eliminate a step and reduce the possibility of paint damage from the strip out process. “After all, he said, doesn’t urethane stick best to itself?” The simple answer to that question is yes. Urethane, like many other adhesives, sticks better to itself than it does to other substrates. There is more to this than meets the eye, however.
Way back when GM instructed partial cut installations—that’s right, they actually gave instructions on how to close cut windshields for the exact reasons given above—there was a belief that saving the paint was the most important consideration and the adhesive would last longer than the vehicle did. However, there were gray areas in their instructions and when the adhesive manufacturers recommended against it, GM updated their directions to accept only the full strip method in the late 80’s and early 90’s.
We all know how to define the full strip method of installation; cut out the glass as close to the underside surface of the glass as possible, then trim the existing urethane bead to approximately 1-2 millimeters (1/16 inch), apply the new bead of urethane directly on top of the existing bead and set the glass. This is what we have been doing for more than twenty years. What interested me was that my caller described the existing urethane bead after a wire out and asked if it was okay to bond directly to the existing bead without a trim back. This question gave me pause and I had to rethink why we trim back the urethane bead.
Over the years I have talked to dozens of urethane manufacturers’ reps and chemists about this very issue and I have really never gotten a definitive answer that carried on from manufacturer to manufacturer. I have literally been told to follow manufacturer directions “because we said so.” I have decided to verbalize what I have been told and what I believe and let the chips fall where they may.
- Urethane cures from the outside in and dies by the outside in. What I mean by dies is that the urethane breaks down into a black caulk-like material when it is exposed to ultraviolet light. No matter how good we are adding black frits, wide moldings and solar glass, the light still gets to the urethane and begins to break it down eventually. When we strip back to 1-2 mm, we take off the older dying urethane and expose the fresh “virgin” urethane beneath, thus making a perfect bonding surface for a long lasting seal.
- The 1-2mm bead we leave acts as a buffer so we do not damage the painted surfaces underneath. It also acts as a foundation that supports the new bead above.
- The more existing urethane left on the pinchweld, the more likely the urethane will deteriorate to the point of failure in a collision by cohesive failure. Fresh, flexible urethane allows for more free movement of the glass in the opening so the replacement of most of the urethane during replacement of the glass is preferable.
Now, to answer my caller’s original question; the recommendation is that the existing bead of urethane is trimmed back to 1-2 mm or a 1/16 of an inch. If the bead left on the pinchweld after a wire-out is larger than that, then it should be trimmed back to meet the recommendations of the vehicle manufacturers and the adhesive manufacturers. If the wire cut out is such that the existing bead is clean and trimmed to 1-2mm or lower, then no trimming is required and that step can be eliminated.
One last thing, I have witnessed some technicians using the braided wire rather than the square stainless wire or solid wire. I have no problem with which wire you prefer, just remember that braided wire leaves debris behind and must be brushed off before bonding.