by Bob Beranek

Photo Courtesy of

I can always tell when a technician was trained by an “old-timer” like me. How? They “spank” or slap the glass into position rather than smooth it in place to make the seal. It comes from the old days when glass had to be spanked and seated into a gasket or when dense butyl tape needed to be forced down to contact uneven pinchwelds. Liquid urethanes call for smoothing the seal, not spanking or slapping the glass into place.

First, let me say it’s not necessarily wrong to slap the glass. It is, rather, ill-advised. There are two negative results that can happen when a glass is slapped rather than smoothed.

  • If the glass is thin, chipped or scratched, then the chance of premature breakage is increased. If the glass is “hot,” the glass is more volatile and can fracture. Some of these issues can and should be caught by the technician prior to installing. However, how hard is too hard to slap the glass into place? It only takes one break to lose your profit and possibly a customer to inconvenience.
  • The second negative issue involves leaking and bonding. Curved glass has a spring to it. It can be depressed as much as a couple of inches before breakage and then sprung back to its original shape or curvature. If the bead of urethane is short and the glass is slapped at its apex, it can make contact and then “string out,” causing leaks and bonding problems. If it is smoothed-out, the urethane bead is redistributed into weak areas, making for a more solid bond and leak-free installation.

If I said I never spanked or slapped the glass, I would be a liar. There are instances where a slight slap is necessary. If you have a large, tall or awkward vehicle where leveraged pressure can’t be applied to the edge of the glass, such as in large service vans, then a slight tap on the top and bottom center may be called for. Of course, this is after a careful inspection of the glass part conducted during the prep stage and that the glass is not exposed to excessive heat.

My advice is to use slapping only when you must and only when the glass was carefully inspected for pre-installation damage. Keep the glass out of bright hot sunlight for as long as possible prior to installation into the opening. Reduce the violence used in past installations as much as you can. The customer will feel less anxious if watching the process and vehicle and glass damage will be reduced to increase profits.


Removing the old glass has always been the most difficult part of the installation process because of the physical labor required. However, the introduction of new hand tools, particularly power tools, has greatly eased this part of the installation. Remember, the goal is to get to the edge of the glass for easy cut-out. Once that is accomplished the rest is easy. To assure that the finished installation is both pleasing to the eye and safely bonded to the metal, here are some recommendations.

—Have replacement mouldings available, whether you think you will use them or not. It ensures that if even if the original mouldings were bent or misshaped, the job will always look good when finished. A good selection of universal mouldings comes in very handy to replace stretched or damaged mouldings.

—Have replacement clips as well. Even though clips are not used as often as they once were, some clips are unique. To make matter worse, many distributors do not stock a wide variety of clips because they don’t sell well and inventory turnover is low. I suggest determining the dominant car dealer in your market and obtaining the clips and retainers necessary to do those brands of vehicles. That way you will have replacements most of the time.

—Never disconnect the electrical components from the rear view mirror while the ignition is on. This could damage the vehicles’ computer or, at the very least, erase the pre-programmed memory. Either:

—Disconnect the wiring harnesses and remove the mirror and place it the back seat; or

—Remove the mirror from its pad and leave it hanging from its wiring harness.

—Pull the cowl panel. In most cases the cowl should be pulled to allow for the best bottom seal. The passenger-side airbag, in most cases, depends on the adhesion of the bottom seal of the windshield to position it properly. It also supports and solidifies the firewall. If the cowl is not pulled, the bottom seal cannot be assured and failure is a distinct possibility.

—When using hand tools for removal, start with the shortest blade in the cold knife and work up to the longer blade. This gives the technician more control of the tool and makes the cut out easier. Look for blades that have extra thinness, serration and coatings to protect the paint.

Use your body weight rather than your upper body strength to pull the cold knife. This allows you work more comfortably and reduces the chance of muscle strains or pulls.

—If you have a part with a rigid or metal-coated moulding system, use a plastic stick vertically to the pinchweld to break the moulding from the glass surface. Then force your cold knife blade between the glass edge and the broken moulding flap and under the edge of the glass. Then pull the cold knife normally. This method cuts out the glass and leaves the moulding in place which eliminates the need to pull the rigid moulding out to access the glass edge. The moulding also protects the pinchweld wall from damage from your tool.

—When using power tools, lubricate the tool by spraying water on the adhesive to be cut and on the blade itself. This reduces the harmful fumes caused by the high speed of the blades and also makes the tool work smoothly. Plain water is recommended over soapy water because it will not contaminate the bonding surface. All cut-out blades are flat on one side and beveled on the other. The flat side of the blade should be to the glass surface. The blade and the glass create a scissor-type action that eases the cut-out. The closer the cutting edge is to the power source the more torque and cutting power it has. So, use the shortest blade possible to cut the material and protect the interior trim.