Avoiding and fixing a known problem with corrosion around the Windshield /Windscreen and the rear window

Something I did a few months after having a new Windshield installed:

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I was reading horror stories about rust-perforated 993 windshield and rear window surrounds due to water pooling in the channel under the black rubber edge molding strip. The moisture also results in ugly corrosion and water leaks into the body of the car. If let go water pooling under the surround leads to an expensive invasive body shop repair. This problem is well understood, is specific to the 993, and became common enough that Porsche added a section covering it in their own shop manual.

Some have proposed sealing the channel by forcing a flexible tube into it acting like a bead pressing against the walls of the channel, My view is this solution will unlikely make a perfect seal and still allows for a void under it for any liquid that makes its way past the beading to cause corrosion. 

Below is a DIY procedure to end the body corrosion issue on the glass surrounds using a sealant on the front and rear glass of a 993. An added benefit to the below procedure is it may also quiet down windshield squeaks that can result from how the windshield was originally set in the urethane mounting adhesive. Also in this document is an additional procedure as to how to repair any spot corrosion that has become visible around the window glass.

Pleae note the 993 has a diffeent glass mounting method than the 964 and earlier 911. 

• The 964 and earlier cars have a single rubber seal that acts like a gasket and suspends the glass in the body opening.  Leaks around the glass is usualy first observed as wetness in the interior lower corners of the windsheild. If you have one of these earlier cars one very effective and easy ways to address a leak around the glass is to force the nozzle tip of a tube of clear 100% silicone bath sealant/calk between the exterior window seal and the glass and the exterior window seal and the body and pump a very small quantity of sealant around the window/winsheild. Any sealant that oozes out can be wiped up and if need be cleaned up with a little solvent. I did this on my '86 Carrera to good effect. The "Cleaning up exposed surface rust" section later in this document will apply to these cars and should be considered before applying the the sealant. I like using the smaller squeze tubes of the sealant as their nozzle is smaller than the calk gun tubes
  
  and can be trimmed to deposit just a very small bead of the sealant. 

• The 993 has front and rear glass that is bonded to the body opening, as most modern cars are today. This is said to cause the glass to contribute to the stiffness of the body. The glass is surrounded by inner and outer molding strips that act as coverings so the meeting of the glass to the body has a finished look. They are not gaskets or seals and the outer one does not do a very good job of protecting the edges of the glass from moisture. The exterior molding strip bridges over a gutter in the body sheet metal that surrounds the glass. The first section in this document describes a procedure to protect and repair corrosion in and along this gutter and outer molding. 

This procedure pertains to the 993 only:

Note: Read the entire below procedure, all notes, and the follow-on section covering cleaning up exposed surface rust from beginning to end before going any further as it may influence how you chose to proceed, what supplies you will want on hand, and/or the sequence of steps you chose to follow;

Supplies required:

• Two tubes of Locktite Roof & Flashing polyurethane roofers black sealant**, for calk gun application and 
  
• A caulk gun;
• A new replacement exterior window molding strip if one plans to replace the existing one. See the note in BOLD on this in the procedure below before purchasing the
  
  molding/seal;
• A can of  "Dust-Off" if an air compressor is not available;
• Some rags;
• Mineral Spirits;
• A 1" plastic paint spatula.

** Others have used  the calk gun applied fast curing sealant marketed for automotive windshield applications such as 3M™ Windo-weld Brand or similar. 

Porsche Technical Service Bulletin (TSB) 9501 specifies Bostik 6050 sealant for this application, something I couldn't find when doing an Internet search for it.

I suspect the above 3M™ gun applied sealant will work just as well. 

I wouldn't use silicone or latex claulking/adhesives that may not provide positive adheason, are incompatible with the existing window mounting adhesive or tend to fail over time.

Also, ribbon adhesive such as 3M™ Window Weld Ribbon Sealer that may not provide positive adheason and may leave voids allowing for water egress.

Here is a listing of  three window sealing products with similar features that also may be suitable for this application. 

https://www.3m.com/3M/en_US/p/c/adhesives/windshield/  

I would not use 3M™ Auto Bedding and Glazing Compound listed on this page as it is a non-hardening formula.

The Procedure:

• In preparation remove the outer molding strip and blow out the channel along its bottom edge with air. You may be surprised by the amount of water that spatters out. You may find that the outer molding strip was previously glued down in places. If this is the case it may get damaged and will need to be replaced once removed.
•  If one doesn't have a compressor a can of camera/computer cleaning dust-off will work well. 
• In my case no moisture blew free indicating things were already dry.
• 
  Don't be too concerned if you see the plastic track the outer molding strip presses into has cracks or broken sections. If it does not adequately hold the molding in place when reapplied a small amount of 3M Super Weather Strip Black Adhesive can be used to tack it down if needed once everything is dry.
  

Note: Be aware you may need to replace the molding strip with a new one once removed. If you do this plan to have a new outer molding strip on hand if you plan to apply it while the adhesive is still wet later in the process. Also, trial fit any replacement strip before beginning this procedure as some available in the aftermarket seem to not fit properly, either not lapping the entire channel or are too short. " From a Porsche TSB on the subject: "When replacing the rear window in cars built before July 1994, a new rubber trim part number - 993 545 225 02 must be used. If the rear window is not being replaced then the rubber trim, part number 993 545 925 00 is to be used. Cars built from July 1994, use the rubber trim, part number - 993 545 225 02."  

 Also, I recently (8/2024) read a posting by Ed Hughes on Facebook

that the aftermarket windshield rubber trim part 993.541.925.00 offered by URO is slightly wider (2.0mm~2.5mm) than the dealer-supplied part and may resolve an issue with the OE trim part not being wide enough to fully lap the gutter around the glass.

If you are replacing the windshield in conjunction with the procedures on this page you might find the section on windshield sources about midway down this page next to an image of a windshield:

https://993servicerepair.blogspot.com/2021/02/words-of-wisdom-page.html

• If you are working on the rear glass and your car has a high-mounted center stoplight that sits flush to the exterior glass the body color matching cover is just held on by friction clips.  
• The cover can be pulled off revealing a nut on each end of the frame holding it to the glass; 
• Push down gently on the frame surrounding the  nuts and remove the two nuts to release the frame; 
• Unplug the two wire connectors to the frame's light fixture; 
• Set the fixture aside, you now have access to the upper section of the glass top molding strip.
• Dust, dirt, debris, or loose corrosion may have accumulated in the channel. To encourage good sealant adhesion any debris should be removed and the channel wiped clean. 
• One can wrap strips of terrycloth around the tip of the plastic spatula or other more rigid plastic or wooden object as a tool for this. 
• Run the tool into the channel moving the tool along it to pick up this contaminant and wipe the sides of the channel. 
• Use multiple strips of cloth as each becomes dirty. 
• Avoid using something hard as a tool, like a screwdriver, as it may scratch the paint or nick the glass forming a propagation point for a crack in the glass.
• Mask off the black plastic track that surrounds the glass so the added sealant calk when applied does not get on it and interfere with the outer molding strip from being pressed back into it when reapplied. 

Note: Some may prefer to address spot corrosion before filling the channel with sealant and replacing the molding strip. If this is the approach used make sure the applied touch-up paint is fully dried before applying the sealant. See the section below "Cleaning up exposed surface rust where the molding strip meets the body:"

• Load the tube of sealant in the caulking gun and slit the applicator tip slightly diagonally to form a moderate size end opening. Then puncture the calk tube sealing membrane at the base of the applicator tip by sticking a wire or nail into the applicator tip.
• With the channel dry, starting at the top center of the glass slip the nozzle of the caulk tube vertically directly into the channel. 
• As the sealant is pumped in it will ooze upward in the channel indicating it is nearly full as one moves along it. 
• Pump the sealant in as one works their way around one side of the window to the center bottom fully filling the channel with the sealant calk flush to its top edges as they go.
• Slow down as you approach the bottom sides making sure the channel completely fills in the lower corners where a surprisingly large amount of sealant will need to be applied;
• Continue to the bottom center of the glass;
• Then begin at the top center again and worked one's way around the opposite side of the car to the bottom center.
• Once dry the masking tape can be removed and the original or replacement molding strip can be reapplied. Some prefer to apply the molding strip while the adhesive in the gutter is still wet using the adhesive to both seal the gutter and tack down the molding strip. If the plastic track the molding strip is pressed into is damaged or has parts missing this is a good way to help the molding stay down. If you apply the strip to the wet adhesive the strip will be permanently affixed so if later removed it will need to be replaced with a new strip.

It may take more than an entire tube of sealant to complete the above process as there is a rather large space in the channel on these cars. Work to minimize any sealant overflow as it will subsequently need to be cleaned off the surrounding body and the outer molding strip once the strip is reinstalled.

Clean-Up after the seal/molding is reinstalled:

• Use multiple dry rags to wipe the excess sealant from the car and the molding strip. 
• Go over it again with a clean rag wet with mineral spirits to pick up any small amount of sealant that was left on the glass, molding, paint, and where the edge molding meets the body paint.

Ideally the above should be done on a clean rust-free channel for example shortly after a new windshield has been installed. I suspect that even if your car has corrosion in the bottom of the channel this process will halt the steady march of future corrosion. Unlike other materials used to fill the channel the above completely fills the void and has extremely good adhesion to both sides of the channel drying to a hard rubber consistency.

How durable is the 993 sealing/calking procedure:

The calking installed by the above procedure has been in place on my daily driver 993 for +/- ten years now on the rear and front glass. It has proven to be durable and eliminates the corrosion issue while keeping the outer molding strip cover flat and in position. Below is a picture of my daily driver 993 taken recently:

 Pictured is the lower right-hand corner of the rear-light glass where I had repaired some surface corrosion that had migrated out from under the molding strip using the method described below. It is a place where rainwater collects and so one of the more common corrosion points on the glass surround on a 993. In the picture, I have pulled the molding strip up.   Note the bottom band of black rubber is the applied sealant in the body channel/gutter. It has become covered with dirt and debris from the intervening years of environmental debris that has made its way under the molding strip. Even so, it exhibits complete sealing continuing to provide complete protection to the body from moisture egress and corrosion. 

Also, note the vertical crack in the top black band, the track the molding strip is pressed into. Cracks or even broken-off sections of this track are very common and should be of little concern. If one finds the molding fails to stay down for any reason including a cracked or broken track, a dab of 3M Black Super Weatherstrip adhesive can be used to tack it down.

Cleaning up exposed surface rust on the 993, 964, and earlier 911 vehicles where the molding strip meets the body:

A follow-up DIY procedure is to clean up any dime or nickel size surface rust that may have already appeared at the edge of the glass. Wait a week or two for the sealant just applied above to fully hard skin over and not contaminate the paint in the below procedure.

Supplies:

• Body-color matching bottle & brush touch-up paint;
• 250, 600, 2000, 3000 and 5000, or 7000 grit wet sandpaper;
• A roll of masking tape;
• A plastic squirt bottle filled with water;
• A one to two-inch painters plastic spatula;
• Toothpicks;
• A "00" or "000" artists paintbrush;
  
• Dupont Perfect-It Ultra Fine Machine Polish or similar;
• Optionally an electric hand buffer is used with a "foam wheel", both available inexpensively from Harbor Freight.
• lacquer  Thinner.
  
  
  

Procedure:

• Pry up and hold the rubber edge molding clear of  the spot of corrosion using the plastic spatula if the edge molding strip is not to be removed ;
• Us lacquer Thinner very sparingly on a clean cotton rag to remove any
  
  residual wax or paint sealant contaminants that may interfere with paint adhesion in the area that you will be working on.
• Use some 250 to 600 grit sandpaper to dry-sand the corrosion down to shiny fresh metal; 
• Use care to not sand the adjacent unblemished finish. Using masking tape one can mask off the surrounding area to avoid sanding it;
• Then wipe and blow off the dust created from the sanding, followed by using lacquer thinner very sparingly on a rag removing any residual contaminants that may interfere with paint adhesion.
• Put a puddle of the touchup paint in a small ceramic or metal dish. Do not use a plastic dish as components of the plastic can contaminate the paint and cause it to not adhere properly;
• Let the paint dry down to a thickened pancake syrup like state;
• Apply multiple layers of the thicked paint using a toothpick or a small "00" or "000" artists brush, allowing the paint to dry between layers until it builds up in thickness above the surrounding unblemished  paint;
• Minimize overlapping the new pant to the surrounding unblemished finish to avoid a ridge from forming in the paint in later sanding steps;
• Remove any masking tape used to protect the adjacent unblemished paint;
• Once thoroughly dry start with 2000 grit sandpaper moving to

  

  
  

  
  progressively finer grits in conjunction with the water bottle to wet-sand the applied paint flush to the existing uncorroded finish that surrounds the repair. A suitable assortment pack of sandpaper of various grits can be purchased on Amazon or any automotive supply store.
• Use care not to sand through the outer layers of the unblemished paint adjacent to the repair;
• Use the spray bottle of water to keep things wet during sanding;
• Once done sanding the process can be repeated an unlimited number of times until the desired result of an invisible paint repair is achieved.
• When done let the repair sit for a couple weeks to gas-out and fully harden before buffing or polishing to one's liking.

I did this repair on my car which is finished in single-stage (no clearcoat) paint. For cars with a clear coat finish the results may vary and larger patches of rust may not match to ones liking. In any event, the above incremental repair will not interfere with the subsequent alternative to overspray the entire window frame, for most a bodyshop activi

CRW from Rennlist asked some insightful questions concerning sealing the window channels:

Q: Once the sealer is in place it's there for good - not easy to remove - correct?

A: If one used sealant from a calk gun, the polyurethane stuff it is pretty much permanent.

Q: What happens if the glass needs to be replaced in the future?

A: If the glass needs to be replaced sometime in the future the sealant can simply be left in place or if any touches the glass in spots it can be trimmed off so I do not see this as an issue going forward.

Q: Does the sealer shrink away from the channel with time and temperature similar to how similar products do around windows etc?

A: In the case of latex calk used on home structures, they gas out over time, shrink and pull away, the stuff used to mount the glass and the stuff used to fill the channel is not water-based latex and so does not shrink over time.

Most every application on a home including roof & widow flashing sealant that is polyurethane, the sealant fills gaps between two or more elements. These elements tend to move back and forth against one another with temperature cycling and so can fail over time. This is not the case with the channel as both sides of it are part of the same contiguous piece of sheet metal.

 Also urethane products have much better adhesion properties than latex or silicone-based sealants. Silicone products are not compatible with the sealaint used to mount the glass and so really should not be used for this application.

If you found this page informative return to the main page and bookmark it for future 993-related servicing, repair, and upgrading guidance & information. If it saved you aggravation, time, effort or some coin consider a donation through the button on the top left margin of this page. 

 

#window #windshield # Windscreen #backlight # Gouge #rust #corrosion #seal #gutter #molding #glass #molding #strip