FAQ > Collision Related Auto Body Repair Knoxville's Finest Auto Glass Installation Center • What are the requirements for properly preparing a vehicle's pinchweld prior to installing an adhesive bonded windshield, side or back glass? The answer to your question would depend on the situation the vehicle is in.  If the vehicle is under going a routine auto glass  installation the answer goes in one direction but if the vehicle is undergoing body work that affects any of the pinchweld areas  that the glass bonds to it goes in another.  The easiest way for us to go there is to tell it all so that in whatever situation you  find yourself you will know the proper procedure.  This answer is rather detailed so in an effort to give the most accurate  answer that we could we went outside the box to I-Car.  Auto Glass Pinchweld Treatment The Wrong Step Can and Will Add A Weak Link A urethane bonded back glass contributes to the structural integrity of a vehicle just as much  as the  windshield pinchweld flanges that hold stationary glass require proper treatment during repairs to insure  proper bonding of the glass and to maintain the structural integrity of the vehicle.  Stationary glass in  vehicles today add to the structural strength and rigidity of the vehicle body.  If the glass is not properly  bonded to the flange, it can alter how the structure reacts in a collision of even during normal body  flexing and driving.  The deployment rate of an air bag can be altered by fractions of a second, where  fractions count, resulting in slightly early or delayed deployments.  Although the windshield is usually  referenced when discussing maintaining vehicle strength and structural integrity during glass installation, proper treatment of  pinchweld flanges also extends to the back lite and urethane-bonded side glass as well.  The focus on this page is the  procedures and concerns that technicians must be aware of when performing both collision repairs and glass replacement as it  relates to pinchweld flange areas.  During collision repairs pinchweld flanges become an issue for one of two reasons. 1. The glass is broken, requiring replacement.  2. The pinchweld flange the glass bonds to will be disturbed due to repairs or panel replacement.  Different recommendations exist for the pinchweld flange when the flange is involved in body repairs or panel replacements,  compared to when just the glass is being replaced.  • Glass Replacement Only When replacing the glass, and the pinchweld flange does not require repair or replacement, the existing  urethane adhesive bead should be left intact until glass installation is started.  This also applies to  pinchweld on the sides of the opening away from repairs or panel replacement.  The reason that the  urethane is left intact until the glass is to be replaced is to protect the base of the bead from  contamination.  Some sources even say that it's acceptable to prime or paint over the existing bead of  urethane during repairs, since it will be cut away when the glass is ready to be installed, exposing a  clean fresh surface for bonding.  Most vehicle and adhesive makers recommend cutting out the glass as  close as possible to the glass, leaving the original urethane bead as thick as possible (see Figure 2).  Then when it's time to start installing the replacement glass, trim the existing urethane bead to a  thickness of 1-2 mm (see Figure 3).  This is a full-cut out method of installations, even though all of the  adhesive is not being removed.  Performing this final cut out just before installing the glass ensures that  a clean, uncontaminated surface exists for bonding.  Leaving 1-2 mm on the pinchweld not only helps  prevent scratching the pinchweld with the cut out knife, it also helps ensure that the new bead will be  located in exactly the same location and size area as the original installation.  • Pinchwelds In The Repair Area If the pinchweld flange will be disturbed, for repairs such as replacing a roof skin, attention to proper  repair procedures are especially important (see figure 4).  This includes using the proper weld  method, and duplicating the number and spacing of original welds.  The structural integrity and  rigidity of the glass opening can be altered by the use of incorrect repair and installation methods.  Once the panel replacement or repair procedures are complete, the pinchweld flange area must be  properly prepared. • PINCHWELD PRIMERS Pinchweld primer is the black primer that is applied to the flange area before laying down the urethane adhesive. The  use of this material is often not fully understood as it relates to the total installation.  Pinchweld primer is a primer,  and like any primer, is designed as a link for adhesion between a substrate and some other material.  The substrate  that pinchweld primer is designed to bond to is OEM paint finishes and some repair paint coatings such as approved  epoxy and self-etching primers.  The other material pinchweld primers are designed to bond to are the  urethane adhesives in the same product maker's system.  Although suggested for use over small spots of  bare metal, pinchweld primer is not a full corrosion-protections package. It provides chemical linking  between the substrate finish and the adhesive that it's designed to be used with.  Leaving the pinchweld  primer out of the process can cause adhesion failure between the adhesive and the flange.  • Guidelines For Pinchweld Primer When using pinchweld primers, some criteria needs to be followed or considered to allow the primer to fully provide the special  chemical link it's designed for.  These are guidelines, and are not intended to replace product-specific or vehicle-specific  recommendations for proper glass installation.  Use only the recommended primer for the adhesive to be used.  Do not mix  systems, as chemistries may differ and failures can occur.  Inside of any one adhesive maker's product line, multiple primers  may exist for use with different adhesives.  Check the shelf life of the primer to ensure it has not expired.  Even though the  primer is unopened, it could be out of date.  Once opened the primer will have a limited time period for use before it should be  properly disposed.  The container should not be left open to the air, as this greatly reduces its working life.  Many primers use  an inner cap and an outer cover to help seal the material from the air.  Some adhesive makers now offer a one-time use  cartridge applicator to help with this problem.  Follow the product maker's recommendations for application.  This will ensure  the correct thickness is obtained.  Directions may be so specific as to state which direction each coat is to be applied relative to  the previous coat.  Do not apply the pinchweld primer on top of any remaining original adhesive that has been trimmed to the proper thickness.  This can lead to a weaker bond when the new adhesive is applied.  Apply the pinchweld primer to OEM paint finishes or on  areas where approved primers have been applied.  The glass adhesive should only be placed where there is pinchweld primer or  an original properly trimmed bead of urethane.  Respect the recommended minimum and maximum dry times before applying  the urethane adhesive.  The primer needs to dry long enough to let solvents escape, however if maximum times have elapsed  the primer may need to be reapplied.  PINCHWELD FLANGE CONDITIONS Some of the conditions that may exist in the glass pinchweld area include corrosion, scratches, dirt, and loose adhesive or sealants. Scratches Scratches in the finish can occur no matter how careful the technician is. Take special care to prevent any type of scratch to the pinchweld that breaks the OEM finish during both glass removal and  collision repairs.  Most glass adhesive systems recommend using pinchweld primer to cover scratches, providing the scratches  don't exceed some size limit.  Pinchweld primers are designed to gain adhesion between the paint coatings and urethane  adhesives, not to bare metal. The best recommendation is to properly apply a recommended epoxy or self-etching primer to  pinchweld scratches.  ALUMINIUM PINCHWELDS Aluminium pinchweld flanges can be affected by using the wrong type of glass adhesive.  Vehicles with an aluminium pinchweld  may require the use of a non-conductive urethane adhesive.  The non-conductive adhesive may also involve another specialized  variable, referred to as "high modulus" (see below).  If a conductive adhesive is used and moisture is introduced into the area, it can pit the aluminium through a process of  electrolysis.  The electrical current comes from antenna and defroster systems in the glass.  As more electronics are installed  into glass for navigation systems or cellular phone reception, this type of problem is likely to increase.  This means that the  pinchweld area can be affected by the use of the wrong type of windshield adhesive.  CONCLUSION When dealing with the pinchweld flange area of a glass opening it's important to keep the following points in mind:  Always  follow the vehicle maker specific guidelines for the make and model being repaired.  Follow the adhesive and paint maker's  directions for selection and use of materials.  Respect all dry times.  Prevent breaking through the OEM finish on the pinchweld  flange whenever possible.  No adhesive can stick any better than what is underneath it.  And when dealing with structural glass  this can become alot more than just a water leak or an annoying wind whistle.  High-Modulus, Non-Conductive Glass Urethane Some vehicles today have glass installed with an adhesive classified as high-modulus, non-conductive urethane.  Although  these two terms are often combined into the same adhesive material, they refer to totally different and unrelated properties.  HIGH MODULUS High modulus refers to a glass adhesive that has specific physical characteristics after it's fully cured.  Modulus has to do with how much pressure or stress it takes to get the urethane to deform, stretch, or give.  High-Modulus adhesive provides a high level of rigidity once fully cured, usually a minimum of 24 hours,  which can help reduce torsional twisting or stiffen the vehicle body.  When used by the vehicle makers, it's  intended to improve noise, vibration and harshness.  Don't confuse high modulus with "high viscosity," which defines the thickness of the adhesive at the time of  application. Viscosity is a scientific term used to compare body while in any type of liquid state.  The higher  the viscosity the thicker the material.  Comparing water to peanut butter, peanut butter has a much higher  viscosity. Urethane adhesive used to install automotive glass has increased in viscosity over the past several  years.  High-viscosity adhesive allows the glass installer to set the glass into the adhesive bead without it sinking or sagging.  An adhesive can have a lower viscosity than another adhesive and still have a high-Modulus rating.  NON-CONDUCTIVE Urethane adhesives contain carbon which can conduct electricity.  Non-conductive urethanes use a special form of carbon that  is non-conductive.  On certain models of vehicles with antenna grids, defroster grids, and other electrical systems integrated  into the windshield or back-lite, the vehicle maker specifies use of a non-conductive urethane adhesive.  This is to ensure that  there's no electrical path to the body through the adhesive bead.  This is of particular concern with aluminium-body vehicles.  If dealing with existing adhesive beads other than the original urethane, non-urethane substrates, bare metal, or non- OEM paint finishes, careful consideration must be given to compatibility with the replacement adhesive to be used and the integrity of the bond that will be formed. Adhesives made for glass installation are designed specifically to chemically bond to the OEM urethane. Using the wrong type of applicator can affect the performance of the primer. Usually a wool dauber-type applicator is specified (see Figure 6).  Wool is used because it will  not react with the chemicals in the primer.  Using an acid brush or shop paper towel can lead to  contamination of the primer.  Shop paper towels can also absorb ingredients out of the primer.  Some primers are packaged as a pen-type applicator to help eliminate this problem and provide  and provide a convenient application system.  Corrosion Corrosion must be removed, and the metal cleaned and protected, or it will re-occur (see Figure 7). This will cause loose adhesion, water leaks, and possible structural integrity problems.  Brushing or spraying primer of any kind over corrosion without removing it is simply buying  time before it re-occurs.  Corrosion that's not removed may also provide a weak link in that  area and allow everything on top of it to separate from the pinchweld.  If corrosion is in  advanced stages, with thinning metal occurring, structural integrity must be carefully  evaluated. Corrosion must be removed and the metal properly treated before reapplying any  type of body primer. Brushing pinchweld primer over the corrosion spot of area will not  correct the problem.   Pinchweld primer should be used over the proper corrosion-resistant  primer only after proper dry times have allowed all solvents to escape the primer.  Loose Adhesive And Dirt Any loose adhesive or other loose material in the location where the adhesive will be applied  must be removed to provide a solid base.  Small irregularities in the surface will be leveled by  the adhesive during re-installation.  Dirt must be cleaned from the flange area so that all  products can gain full adhesion.  It's best to clean dirt from the flange before final trimming of  the existing urethane adhesive.  Avoid solvents when cleaning the flange area.  A clean, lint-  free towel and distilled water is recommended.  Avoid tap water.  Tap water can contain  chlorine, iron, and other minerals that might affect the bonding area.  FOLLOW RECOMMENDATIONS Makers of glass adhesives combine high-modulus and non-conductive into one adhesive because the vehicle models requiring one characteristic generally require both. This type of adhesive should be used to restore a vehicle to OEM specifications when service procedures specify or when the vehicle was made with it. Using high-modulus adhesive when it's not specified or when it was not designed into the vehicle could alter the vehicle, making it stiffer than original causing other potential problems. Many vehicle makers and adhesive suppliers are providing guidelines on proper cleaning and scuffing for  preparing the surface for epoxy or self-etching primers.  Follow the paint maker's recommendations for  proper application of the approved primer being used on the flange.  After the proper dry time and before  applying a topcoat, the area of the flange where the urethane adhesive will be applied must be masked  off (see figure 5).  Some paint systems may recommend doing this following the sealer applications  rather than at the primer stage.  The flange area that requires masking is primarily the flat portion where  the adhesive bead is to be applied.  The area beyond where the bead is applied, moving out of the flange  area, should be top coated for corrosion protection.  Primers must be fully dry, allowing all solvents to  escape, before beginning the glass installation.  Do Not!  use body filler on the pinchweld where glass urethane adhesive will be applied. Small irregularities in the flange surface will be filled when the adhesive bead is applied. 2.  Do Not!  Use “Spray Can Etching Primer” on the PinchWeld. It WILL NOT! dry to the point where the windshield Primer will not emulsify it.  There maybe some Brands out there that may work but in our experence we haven’t seen one. The Typical Spray can Etching Primers we have seen used have never worked.  “When you decide” to take a chance and use  them anyway it ends up costing you valuable time as well as the moneys involved in materials used in striping the pinchweld  surface back down to the bare metal surface and starting over again.  In our mobile operation, it costs us a second trip which  unfortunately we have to pass along to you in the form of a service fee adding even more expense.  Almost all of the information on this page was taken from an I-Car article Dated March - April 2000.  You can see the original article at the link below.