The Porsche 911 993 windscreen is one of the most uncomplicated and straight forward windscreens to fit, yet they can give you many problems if not done correctly.
It’s fairly common knowledge that the two most prolific issues which arise from the windscreen, or windscreen area of the car, are corrosion of the aperture (or pinchweld) and a very irritating vibration – or creaking – noise. Conceding that the former could well be attributed to a design flaw, both problems are usually traceable to human negligence, especially if the car has had its windscreen replaced. To identify and illustrate what these problems are, and just how bad they can get, I have chosen this 1995 car on the basis of its age, condition and comprehensive service history.
This 993 is extremely ‘clean’ and looks to be a fantastic depreciation-proof investment. A C4 coupe 6 speed manual, she’s flawlessly finished in a Polar Silver metallic paint with an unmarked – and as new – gray interior; hard backed leather sports seats to boot. The car has been well maintained and between just three former keepers every mile, every turned nut and bolt, has been meticulously accounted for. The current owner wanted to address two annoying problems which first began to surface a few months after he’d bought the car: corrosion (on the scuttle below the windscreen) and, “it sounds like the pitter patter of rain” he said.
NB: I have covered the VIN in order to concentrate on the subject, and not on the car or its identity.
Like with any windscreen replacement, most car owners will see the finished product; in most cases, they start with a damaged window and when the work’s done, they’re presented with shiny new screen. If there are any issues in the finish, sometimes they’re immediately obvious but some problems will not manifest themselves until later. One of the most annoying scenarios is finding such a problem as the new owner of the car. The story with this car started with such a sketch.
To be fair to the current owner, it’s an easy detail to overlook when looking over an otherwise fantastically maintained – and presented – car. “It was only until after I got it home and wondered if it should be like that,” he said “I decided not to query it as I was so pleased with the car”. Some six months into his ownership, his ear ‘tuned in’ to the infamous creaking noise and like many other Porsche owners, he put it down to a dry bush or guessed that it might be a tightness in the dashboard somewhere. The car subsequently ended up with a Porsche independent who immediately spotted the gap in the damming rubber, which prompted him to look for other telltale signs. On closer inspection, there was no doubting the windscreen had been replaced. Underneath the outer trim, or ‘sealing frame’ was not a pretty sight.
Please also note that there may have been approximately a year (or more) between spotting these issues and having them corrected; the bubbling paint was not as bad when first noticed.
My remit was to remove the windscreen so that the bodyshop guys can tackle the corrosion. The following pictures tell their own story.
The windscreen was removed – intact – and whilst it was difficult to ascertain if had been fitted from new (or removed and refitted) there was no mistaking the attempts to cover up a multitude of mistakes.
Leaving the bodyshop guys to deal with the corrosion (which pointed towards some over zealous use of a knife by a previous installer) the windscreen needed some extensive correcting of its own before it would be ready for refitting. My suspicion is that for whatever reason, the windscreen had already been removed and refitted. This may have been in a bodyshop (for paint.corrosion correction) or possibly even to rectify a replacement problem such as creaking, leaking or wind noise.
The Clean Up
There was a lot of making good to be done and found hidden beneath the mess was one of the reasons why these windscreens creak: silicon contamination. In order to obtain a more flush-fitting finish, the 993 front screen comes, from the manufacturer, ‘pre-encapsulated’ within the trim frame (like the one on the heated rear window, this is not available as a separate part). The plastic-coated frame is slotted onto the glass and provides a groove for the outer (rubber) trim to lock into. With the trim frame around the perimeter, a bead of Polyurethane adhesive (PUR) is applied over the line where the trim frame meets the glass. The idea is to bond these two parts together and in doing so, provide a raised profile to which fresh PUR is applied when fitting the windscreen to the car. A mistake many make is to cut into this pre-applied PUR. In this case, there was little left of the factory-applied PUR; the silk-printed ceramic frit had been obliterated with black adhesion promoter and the trim frame was loose. Given how contaminated the substrate was, the best policy was to cut right back to the ceramic coating; much of what was left of the existing PUR simply peeled off.
The next steps were crucial. Nobody likes doing preparation work; it’s boring, it takes time and you cannot see an immediate result. However, the importance of prepping the substrate sufficiently couldn’t be more important in this case. This screen was riddled with silicon and had to be removed before we did anything else.
With the contact surface neutralised, work began on getting the trim frame back on and bonded under fresh adhsive. There are a couple of points which I am purposely omitting at this stage under the banner of trade secrets but mainly because this is a very involved remove and refit of a 993 screen and that those particular details would probably not apply to the fitting of a new screen.
With the fresh PUR applied, the windscreen is left in a cool, dry room to allow the adhesive to cure through to its core (brand, temperature and humidity dependent, this could take up to 84 hours). For your information, I use a Sika AGR OEM Approved PUR adhesive system. Once the PUR has cured, some trimming may be required to allow the screen to sit at the required height.
The Repainted Car
From the moment the fresh paint has been baked, the car must be allowed to cool down for 24 hours after it is removed from the oven. This is part of the paint curing process. Sika recommends a bonding process specifically for freshly painted cars and this process is followed according to their guidelines.
With the inner damming rubber in place, there are some minor adjustments to be made before the windscreen can be bonded back into place. These tweaks are made from ‘dry’ fitting the screen and ascertaining the where correct position is in relation to where – and how – the screen will sit on freshly applied PUR. Some final preparation touches are made before setting the screen into position. And she’s in:
The final step is to make sure this problem doesn’t creep back in years to come.
Some shots of the finished job.
I invite, and welcome your comments.
Brilliant write up and pictures describing 993 windscreen installation (and problems) to a tee.
Most people get these horribly wrong, and this example isn’t a rare one unfortunately. So it’s refreshing to know that there are still some technicians out there with some integrity.
Well done !
Great write up! My only question comes at the very end. I read the whole blog and then read:
“I do this by filling the gap with a non-curing product, similar to Dum Dum (and for all you windscreen techs, no, not Arbomast!).”
Why are you being opaque about it? What product do you use?
Opaque, and somewhat secretive because a) it’s not really a DIY job, and b) I’ve put a lot of time and effort into researching this particular subject and whilst I have shared it with several trusted installers in the UK, I’m not prepared to furnish other installers with this information.
If you have a 993, which has not been backfilled, I will gladly supply you enough product to do the job.
Paul, I’m about to take my car to a shop in Seattle, how can I get some of the backfill product?
you can use 3m bedding compound(has a dark graphite almost metallic look), same stuff I use for old rubber gasket cars, especially mustangs. You can also use a low viscosity primer-less urethane, but it will be a pain for the next installer. If the primary retention bead is correct there is no reason to back fill, Porsche didn’t think so anyways. This windshield is no different than a honda civic, in fact It technically easier.
Thanks for your comment.
I find that a lot of installers are applying the same thinking to fitting a 993 windscreen, as they do with other cars. Much is lack of experience but in my opinion, there is a distinct lack of understanding about what the potential pitfalls are, or why these (common) problems occur. One of those issues is water pooling in the bottom corners of the recess; the backfill takes care of that problem and is another demonstration of how a replacement windscreen can sometimes be better than the factory fit.
How do you deal with the antenna. It seems inaccessible
Deal with, as in disconnect it, or retrieve it to prevent damage when cutting out the windscreen?
I figured it out, to unplug and re-plug you have to reach deep into the hinge pocket of the frunk. Either have a small child handy or some long needle nose pliers and good dexterity.
Yes I got a 993 last year and it is creaking a bit from the windscreen.. I can live with it But should I be worried ????
I wouldn’t worry, not until you’ve had it looked at. Creaking usually means there’s movement. Movement usually indicates a fitting issue…
Paul, I have just bought a 993. It has some rust around the bottom of the windscreen as well as some creaking noises (couldn’t pinpoint where these we’re coming from). What sort of money are we talking about for the repair you’ve described please? Would you be able to take a look at the car and give me your views? Thanks Martin
Great write-up. Wished I had read it before purchasing my 993.
My new-to-me 993 is now in a body shop to (initially) repair a small rust spot in rear window. But because the front and rear window trim frame plastic have all but disintegrated, I now need to replace both windows since trim frames are not sold separately!
What began as a $500 job is now costing me in excess of $2000, not to mention the grief from discovering that the car has had a complete respray, contrary to the information that I was given by the mechanic who recommended and did the PPI.
Agree with the others, great write up!
What if the plastic frame around the windscreen ( that the outer seal is fitted to) is damaged.
A little cracks from age I guess.
Is the only alternative to buy a whole new windscreen??
That is what my body shop is telling me..
There is nothing else wrong with the windscreen.
Seems like a waste.
Hi Marcus, and thanks for your comment.
The bodyshop are correct about the trim frame: it comes with a new windscreen. For this to be damaged it’s either age (the coated metal corrodes) or there has been some careless hands on it at some stage of its life. It’s not inconceivable to replace the trim frame (it would have to be a secondhand offering) but it would involve quite a bit of work. I’ve done it (and do keep a stock of trim frames) but it is very time consuming – the subject car would also be without its screen for the time it takes to rebuild the assembly (at least several days).
I’ve also managed to repair them in-situ, but only on the slightly damaged ones.
MY REAR WIND SCREEN IS IN WORST SHPE THAN MY FRONT,STILL THE FRONT IS STARTING TO PIT AT THE EDGES.ANY TO STOP THE WATER FROM COLLECTING AT THE RIGHT REAR WINDOW?
I am very interested in your back fill product. My 95 has problems front and back. If I use it now, will it need to be removed when I have repairs done? Can you ship me some to do front and back? What would you charge?
Please let me know, I live in northern Mi. far from any Porsche repair shop.
Very good Paul I have been installing high end windshields for over 40 yrs . The problem Is that when you push the outer seal into the plastic retainer before the urethane is set up , you end up pushing the windshield in to far. Unless you babysit the car until its dry. That being said how are you sure the highth is correct when it has dried . This has been A problem when installing a new outer seal before its dry because it will push the windshield up because it is new and curled under . I wood like to hear from you Thank You. Pat… Fellow Glass Tech.
Hi Pat, and thanks for your comment. YHM.
Paul I made an error on my previous post I meant
You would push the windshield IN” if the urethane is not dry . Thanks Pat.