My car arrived to me with the windshield and old seal in the back seat. I re-installed the old crappy seal after scraping off the old sealant with the 3M glazing compound just where the seal will lift off the outside all the way around it.
3 years later, no leaks.



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That 3m glazing compound is what I have been experimenting with on another car, and am ready to try on the 911 when I do my next reinstall test this summer. Maybe I even got the 3m recommendation from you?

I should also mention that in my testing, my car has been exposed to +80 degree weather in the summer and -40 degree weather in the winter (inside an unheated garage).

Not everyones experience will be the same as what I am seeing with the rubber seals cycling through these extremes.

Also, I have purposely left my interior unfinished as I experiment so I can detect even minor leaks.

What I am seeing with my latest install (still no sealant but apparently a good dry install) is that if I look really hard I can find tiny minuscule droplets that from inside the car midway down the inside edge of the rear trim (where the rubber seal meets the side panel that is covered with headliner). By the time they get to the bottom of the glass near the parcel shelf they often have dissipated and leave no pool inside the car on the parcel shelf, but do leave moisture inside the seal.

If I leave the car out in the weather for days, the water that pools outside on the lower rear corners of the glass eventually seeps past the seal and onto the parcel shelf (normally hidden by carpet). As the new seal ages that will get worse.

With the design of the rear glass in particular, I do not trust the seal to stay tight and dry, giving the design of the rear window and these external naturally pooling areas on top of the glass (at the lower corners) that just don't dry up on their own until things really warm up or you soak them up from the outside of the window with a rag. Those pools are deep, and due to the high humidity in my area, they even form on the glass some days (mornings) when it doesn't rain.

At this point I have concluded that as Timmy2 suggests, using 3m glazing compound or similar product to act as a flexible, thin, non hardening membrane between the seal and the glass, or the seal and the body is the ounce of prevention that these cars need if they are regularly used and exposed to the elements. I see no down side. It can be cleaned up more easily than urethane sealant and will adjust with the shrinkage and expansion of the seal with weather or over time.

I see no reason not to use it on the front windshield as well (unless you want to use a more permanent glue to stop movement).

Aside from being a little bit messy (I plan to mask my Alcantera dash during install) it helps the seal slide into place more easily.
As I researched this product I also came to the conclusion that for people that aren't ready to replace their seals, they could mask off their window and painted areas (easier cleanup, but it wipes right off anyway with mineral spirits) and apply it as Timmy2 describes. Maybe the old seals can live to work another day without replacement.

Here is the picture thread again. Pay particular attention to the picture of the black 964 if you think that it is all about the rubber seal.
(Yes this is a scare tactic).
http://forums.pelicanparts.com/porsche-911-technical-forum/686953-why-windshield-trim-should-regular-maintenance-item.html

P.S.
There are other opinions for various reasons and I respect those, so please keep the discussion going if anyone has anything to add. I am anxious to explore this further.

This how-to article was just posted by 3litre on another thread here.

They use the same non hardening 3M glazing compound

I am adding it here for future reference

Window Glass Install - Modified Mustangs & Fords Magazine