THE KNOWLEDGE

All 911s up to and including the 964 (and many other cars of the period) have their windscreen and backlight both secured and sealed by means of a rubber extrusion, into the outside face of which is pushed a filler trim. This trim must first be pulled out to remove the windscreen, complete with the rubber, but it is easier carefully to cut the latter unless it is essential to use it again. In order to install the same type of window, the extrusion (new or used) is slipped over the perimeter, the filler strip inserted, and then the entire assembly eased into the rebate with a length of strong cord. This is looped around the lip on the inside face of the rubber, and used to pull it over the metalwork. For more on this see the March 2014 issue, in which we followed the process on a Carrera 3.2.

From the 993 onward, all 911 front and rear windscreens – and those of virtually every other car, whether Porsches or not – are bonded into position, although as you might expect of the 993 the design is a kind of halfway house, with much the same style of rebate as in the 964 and earlier models. The 944, the 968 and the 928 have bondedin windscreens, and perhaps surprisingly for their age this is how both the 924 and the early 944 with the ‘recessed’ windscreen have the glass attached. All windscreens of this nature can be removed only by cutting through the strong combined adhesive and sealant that essentially glues the glass so firmly into position that it becomes an integral part of the vehicle’s overall structure.

In the 993 this requires the removal of the outer and inner rubber trims (which might be reusable, although it’s best to buy new), and the air-vent trim on the top of the dashboard. The wipers have to come off, too. (In many more modern cars you also have to take off the scuttle cover below the rear edge of the bonnet, and much else besides.) This allows essentially a long needle to be pushed – very carefully – through the adhesive from inside the car, and the eye at its sharp end to have passed through it a stainless-steel wire. The wire is thus pulled back into the cabin, and to each end is attached a ‘T’-handle via which it is used, cheese-cutter style, quickly to slice through the remaining adhesive. It requires considerable strength, and care to avoid damaging paint or slicing through any adjacent trim, but is in truth simple and effective. Some fitters use a vicious-looking angled knife, but Paul Ralhan resorts to that only in dire emergency. ‘It’s just too easy to do a huge amount of damage,’ he argues.

Once the glass is out, the remaining adhesive can be scraped out of the rebate – at the same time minimising additional damage to the paint and underlying metal; Paul uses well-worn and suitably rounded blades – and then the condition of both that paint and the underlying substrate can be assessed. The blue car shown here had previously had a new screen fitted rather poorly, and was showing signs of corrosion that Paul decided would have to be dealt with by a bodyshop. In cars with just a few rust spots, however, he scrapes and sands the affected areas back to bare metal, and applies first an etch-primer and then an activator for the adhesive. Between them these will stick so firmly to both paint and any ‘bare’ metal that moisture is excluded, and thus prevented from generating further corrosion. The good paint is roughened slightly with an abrasive pad to provide a key for the activator.

The new glass comes with a bonded-on rubber edge. Cheaper (or slightly cheaper, anyway) after-market windows don’t always fit as neatly as OE items, suggests Paul. The inside face of this rubber often has to be trimmed back with a sharp knife in order to allow the pane to sit down at the right level within the aperture, and allowing for the thickness of the adhesive. It is essential to take off no more than is necessary, although any slight shortfall can be made up for with extra adhesive. The 993 glass also has on its exterior edge the hard-plastic channel into which will be pushed the locating flange of the rubber outer covering trim, for the time being filled with a protective filler strip.

The next stage is to fit either the new or original interior perimeter trim, pushing it firmly into position all the way round the flange on the rebate. In the 993 this will also require the cutting of a slot in the rubber near the bottom right-hand corner of the aperture, through which is passed the wire for the radio antenna. After that comes a trial-fitting of the glass, making sure not only that there is a uniform gap all the way round (usually established by temporarily placing packing strips beneath the lower edge, if necessary) but also that it sits at the right height relative to the roof, the pillars and the scuttle when viewed from the side. This is essential to make sure that the outer trim sits correctly. Paul can tell by eye alone whether or not it is right, but checks with a ruler.

After that comes the most impressive, even magical, part of the process. The glass is lifted back out, being careful not to disturb the packing pieces, and then, using a batterypowered version of the traditional skeleton gun, Paul quickly and confidently runs a bead of adhesive all the way round the rebate. And not just any old bead. A triangular slot cut in the end of the nozzle gives a precise ‘blade’ of adhesive, with its broader side against the metal, and the sharp edge facing the window, and the instant-backwind function when the switch is released allowing the two ends to butt against each other so neatly that the joint is invisible. For both of the front screens Paul placed the bead on the body; for the blue car’s rear screen he placed it on the glass. ‘It’s just easier that way,’ he said.

The next step is to offer the screen back into position for the final time. The adhesive takes about 20 minutes to start going off, and roughly an hour to set fully (which is why you have to leave the car standing for at least that time), but plainly you don’t want to have to disturb it once the bead has touched the rubber. So while there is a little leeway to reposition the screen against those packing pieces, for instance, it is in reality a bit of a do-or-die operation. And in the 993 it is slightly complicated by the fact that you must simultaneously feed the antenna wire through that pre-cut hole in the rubber trim, but Paul accomplishes that with practised ease – and clearly a right arm strong enough to hold the glass in rather a precarious-looking position while he does so.

Finishing touches now: filling the gap between the hard-plastic channel for the outer trim/seal and the body with more adhesive, and smoothing it off with a solvent-coated spatula; fitting that outer trim itself, making sure it pushes down smoothly, and at the correct angle relative to glass and body – and perhaps using a clever suction-pad lever device gently to push down on one side of the glass to compensate for a slight discrepancy of fit, possibly the result of a minor distortion in the body shell as much as a fault in the glass. ‘But I only ever use that if I have to,’ says Paul, ‘because if you put a permanent “set” into the glass there is a danger that it will sooner or later crack. And if you have to push down really hard then there is clearly something very wrong somewhere.’

Wiper arms back on, dashboard trim, much cleaning and wiping with special solvents to remove every last trace of adhesive from where it’s not meant to be, and that’s about it. Not quite as dramatic an effect as having a complete respray, but for any car with a windscreen that has done more than about 50,000 miles the next best thing. A bit like a new pair of spectacles – or even just cleaning the old ones. Suddenly, you can see clearly again!