What is a “plunge relief cut”?   I see this procedure used all the time and every time I see it, it makes me cringe.  If completed incorrectly or used more often than not, it is a sure cause of corrosion and bonding failure.  The plunge cut is when the utility knife is plunged into the urethane bead at the glass edge and run along the edge to make the cutout procedure seem easier.  I use the term “seem” because it is an incorrect assumption.  If the cut out blade is sharpened and used correctly the need for plunge cutting is rarely necessary. 


The problem is not that the metal is scratched – we scratch the metal all the time – the problem is where the metal is scratched.  If a scratch occurs in an area that can be seen and addressed, it can be corrected with the proper procedures and preparations.  But, if it is scratched below and under the trimmed existing bead, the chance of properly treating that scratch is very slim and undermining corrosion is the result.  If the recommended amount of existing bead is left after trimming (1-2mm) and a scratch was caused under that bead, the only way to prime or cover that scratch is to expose it enough to allow the primer or prep to flow to the scratch.  This will mean that you will either have to trim the urethane to bare metal – which will cause further corrosion treatment – or you would have to use tweezers to spread the existing urethane bead to allow for primer flow.  If you fail to do this, not only will the installation probably leak but the total bond will be corrupted and eventually fail without expensive correction.


I’m sure that some of the readers out there are saying,” Beranek, don’t you know anything?  There are times that I have to plunge cut.”  Of course you do.  I do know that, in some instances, it is a necessary evil to plunge cut.  For example, when poor prior placement of an existing bead oozes and flattens out or when the need to trim off encapsulated or bonded moldings is required.  Of course it is necessary to plunge the knife blade in those instances, but the key is not to plunge it too deep that the metal will be scored and access to the scratch is impossible.


The best way to deal with plunge cutting is to not do it.  The best technicians are those that spend the time and/ or develop techniques that eliminate scratches to the metal.  It takes time, practice and possibly new tools to accomplish this goal but it can be done.  Please, don’t develop bad habits that “seem” to make your job easier but ultimately damages the vehicle and endangers your customers.