A question often asked amongst motoring enthusiasts is, “I’m having my windscreen replaced today. Is there anything to look out for?” Generally, the advice will be pretty good, and mostly based on experience. However some responses are the work of fiction; a fallacy; an internet fable, see it here.
“Before the fitter puts the new windscreen in, make sure he paints over the scratches with that black primer!”
Paintwork scratched by windscreen fitter.
This statement highlights an important issue, and just how easily vital information can be grossly misinterpreted and misrepresented. Three of the most used aftermarket polyurethane bonding systems used in the automotive glass replacement ( “AGR” ) industry in the UK are Betaseal (Dow Automotive) Dinitrol and Sika. All three are also used by vehicle manufacturers on the assembly lines all over the world. From their respective websites:
“Black, solvent based physical and chemical drying primer designed to prepare the cleaned automotive glasses and the flange for direct glazing. DINITROL 530 promotes the adhesion of the polyurethane sealer to the substrates, but also prevents the degradation of the polyurethane due to Ultra Violet radiation.”
“BETASEAL 43532 primer is recommended for use as an adhesion promoter between BETASEAL™ urethane adhesives and automotive paint coatings”
Sika 206 G+P
“Sika® Primer-206 G+P is used to give improved adhesion in adhesive bonding applications on sub-strates such as glass, ceramic-coated glass and painted surfaces. Sika® Primer-206 G+P can also be used on other substrates such as plastics and some metals.”
The product descriptions are condensed, and a fuller technical data sheet furnishes us with all the finer details, but notwithstanding, the single paragraphs – at a glance – tell you what you need to know. The common denominator in all three: black primer is an adhesion promoter. Applying it provides a suitable substrate to bond to. There is no mention of it being suitable for coating bare, or bared, metal. The emphasis of its use – by its users – is largely misplaced, and anyone using black primer – adhesion promoter – to paint over scratches is missing the point: avoid scratching the paintwork to begin with. The long term damage can be very expensive and often does not manifest itself until the car has moved on to a new owner.
BMW E46 M3 Windscreen Removed
None of us are exempt from making an innocent mistake, but when we do, we learn from it; we gain knowledge. Experience is knowing all the pitfalls of your trade and more importantly, how to avoid them. We don’t paint the mistakes black with primer. We consciously avoid making those blunders in the first place. The prevention is a critical part of eliminating the risk of corrosion. Like in the image of the BMW E46 M3 above, sometimes there is no excuse to scratch the pinchweld on removing the windscreen and there is no justification whatsoever for the pinchweld to be bared of its paint when cutting back the old Polyurethane ( “PUR” ). Steve Allard of Western Windscreens in Launceston shows us just how futile covering a scratch with black primer really is. The car: a Mercedes-Benz SLK had come into his workshop for a new windscreen. It should have been a routine replacement, but the job turned into much more when he found some scratches which had been hidden with something that was not designed to protect bare metal from oxidising.
Picture courtesy of Western Winsdcreens in Launceston.
Nobody in their right mind could claim the car is a rot box as the metal around the scratch has not rusted, therefore negating the claim or suggestion. The black primer is clearly not sufficient, and does not inhibit the corrosion of the metal beneath the paint.
Image courtesy of Steve Allard, Western Windscreens, Cornwall
Black primer is not for painting scratches. Its use is for preparing a freshly painted surface to promote better adhesion; it is used to provide a better a substrate to bond to; it is not designed to be used as a rust inhibitor, or to ‘fill in’ scratched paintwork. Even accident, or crash repair centres can be guilty of thinking that the windscreen fitter’s black primer is some magical cure, and an excuse for the painter not to put in the effort:
Unsuitable for bonding
A Porsche 996 (pictured above) had a new roof skin replaced prior to the windscreen being removed after it cracked on a track day. Cutting back the PUR revealed some pretty alarming evidence of what was supposed to be a repair by a Porsche ‘approved’ bodyshop. There was evidence of black primer under the PUR but it had not adhered to the metal at all, and just lifted off the pinchweld like the floating thin layer of brittle black primer that it was. There was no chemical bond between to the two materials.
Those who are engaging in such practices will not be around to witness the damage in time to come. The car may even have since exchanged hands, and thus voided any warranty however subjective it may appear to be in the circumstances. The perpetrators of such careless and shoddy workmanship cannot be held culpable for the consequential loss which will invariably have been caused by their negligence, and this is as much of a crime as the act itself.
Removing a windscreen, and cutting back the adhesive that held it in is easier today than it has ever been. There are a variety of removal apparatus and cutting implements readily available whilst finely honed techniques remain the most important tool of the fitter’s repertoire. Yet there are so many who neglect to use them or are ignorant to the fact. In some ways, the windscreen fitter skills and mindset has perpetuated its own paradox. Added to this is an incessant need for speed; there appears to be so much emphasis on cheaper, easier and quicker ways to get the job done. Not many have the time nor the inclination to do a good job, or take pride in their work.
Appropriate and correct treatment of bare metal
It is inevitable that at some point, you are going to bare some metal as you cut back old PUR. It might be a raised spot-weld, or it might be a slip with your chisel. Within reason, and after careful assessment, you may be justified in ‘dotting’ a small mark in the bondline with primer, but if you want to be thorough, and offer a guarantee which is as robust as the work you’re aiming to complete, or is in line with the car manufacturer’s warranty, you have to spend a little bit more time than just dabbing on something which is not fit for purpose. A little more application – literally – is in order.
Treating bared metal with a chemically bonding agent
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