Replacing Back Window

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Replacing Back Window

Post by Mcarlo77 on Tue Apr 05, 2016 8:01 pm

Here is a subject I don't recall having been addressed on this forum.  With Spring weather nearly upon us, one of my projects at hand is to remove and reseal the back window on the Laguna.  I know I put the cart before the horse when I re-finished the trunk last summer without dealing with the leak, but it just seemed easier at the time (and, less daunting) to tackle the trunk rather than getting in deep with removal of glass, cleaning/repairing of pinch weld and re-sealing the glass.  Time to face the music...

I've just spent a couple of hours reading threads on other forums (Team Chevelle from several yrs ago was most helpful) on how to properly do this job and my head is spinning.  Some say use butyl sealant like they did at the factory.  Others say the factory never used butyl...rather, some sealant made by Thiokol that flowed similar to urethane while utilizing a "dam" to help contain it from oozing.  Some say glass shops can't legally use butyl anymore and use urethane...which can be messy and squeeze out to form unsightly ooze around the trim inside the cabin.  It's insinuated that installers aren't trained to be careful with that since the newer cars it's used on has that black perimeter that hides the squeeze-out.  It's like asking people which shocks to use...everyone has a different opinion.

So...I'm opening up this discussion to find out exactly what any of you have done when preparing and re-sealing a windshield or back window.  How do you prep the pinch weld area?  What size/thickness/manufacturer of butyl (if that's what you use) do you find works best that allows the glass to sit at the right height for ease of trim installation...yet, seals against leakage?  Likewise, what are the issues/pitfalls of properly using urethane sealant?  For instance, how to use closed cell sponge or foam as a "dam" to prevent oozing.  Blocks required to keep the glass from sinking too far down...preventing proper installation of trim.
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Re: Replacing Back Window

Post by Joe73 on Tue Apr 05, 2016 9:04 pm

I've done at least 5 windshields and 3 back windows on G3s back in the day.

As for blocks to prevent sinking I have used blocks but that was at the bottom of the glass so it doesnt SLIDE down too far and stays in position.

I've never run into a car with pinch weld damage. I just clean the pinch weld area real good with some lacquer thinner or such. Nothing mineral based which might effect adhesion of the Butyl tape. I clean the glass good, install the tape around the edge, place the blocks at the bottom and install the glass. Then I use some windshield chaulk (dont remember what the stuff is exactly) to seal around the edge just in case.

I've never had any leaks or come backs. My windshield I removed from my chevelle during tear down was just sitting there with butyl tape. Nothing else. And it didnt leak.

I believe there are currently 3 thicknesses of tape but I'd have to search around to find out which we would need for our cars. Ive heard that current glass manufacturers are making the glass thinner than back in the day. So I guess you can actually measure the glass thickness, put one piece of trim on so it sits flat and then measure the gap. Some simple math should give you the tape thickness needed so the trim will lay flat when installed.

I would guess there is a better way nowadays to install glass but I'll be doing it the same way like I've always been.

Good luck with it.
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Re: Replacing Back Window

Post by bitterfitter on Tue Apr 05, 2016 10:00 pm

If it matters , I just installed all the glass in my car in the past month or 2. I have done all these in the past with 7/16 butyl tape and never had an issue. Windshield has places to rest when installed and so do the opera Windows. I used tongue depressors tape together for the rear glass. I use them on the bottom and the sides to make sure it's centered.
The set of glass I just put in I used 3M urethane and I have to say it was really nice. I may never use the tape again. The tape tends to follow any imperfections in the body like welds and seams. The urethane can be applied as thin or thick as needed and levels great when the glass is installed. I laid just enough to hit the glass a squeeze out slightly. I had one area on an side window where it squeezed out a little. I just let it dry and used a new razor and removed the excess .
The urethane holds the glass really good. Even when just touched together, it held my side windows in place but allowed me to slide them slightly in different directions to get them just right. Not inches but like maybe a 1/4" at most.
I think the butyl is easier if you are unsure but the urethane is a much better seal.

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Re: Replacing Back Window

Post by pila on Tue Apr 05, 2016 10:34 pm

When I did the windshield and rear window on my Elco, I used the sticky rope type seal. I believe it was like 3/8" or 7/16" stuff. After that sat a day or so, I went around the edge of the glass itself, like Joe mentioned, with`some sealant (that I forgot the name of !)
Sealing the edge of the glass is a good idea in my opinion, since I've seen leaks where the seal became separated from the glass on some windshields...


Bill

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Re: Replacing Back Window

Post by Mcarlo77 on Tue Apr 05, 2016 11:37 pm

So, Joe...you didn't prime the bare metal of the pinch weld before installing the butyl tape? What about using an epoxy primer to thwart future rust?

I do recall discussion about new replacement glass tending to be thinner than original. Seem to recall these other guys were going with 5/16th thk butyl with original glass. Then again, they were working on pre-'73 Chevys...and, maybe the channels aren't as deep? Unless someone remembers exactly, I guess taking measurements is required. But, how much to allow for compression...1/8"?

There was also some discussion about sealing the edges once the glass was set with the butyl tape. One guy said he used urethane sealant for this...but, others were quick to point out butyl and urethane aren't compatible and will eventually separate and cause leaks. Also, not to fill the channel...just the edge of the glass. Anybody else maybe know what the correct material would be used for this?
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Re: Replacing Back Window

Post by Limey SE on Wed Apr 06, 2016 1:10 am


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Re: Replacing Back Window

Post by Limey SE on Wed Apr 06, 2016 1:12 am

Watch that little Video it shows to use Urethane and urethane primer first and also paint the inside edge of the glass to promote adhesion of the sealer to the glass, About halfway thru also Shows a dash Repair I may try Before using that dash cap we talked about Steve

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Re: Replacing Back Window

Post by Joe73 on Wed Apr 06, 2016 12:27 pm

I did not prime or put anything on the pinch weld. I havent encountered any rust issues there. But if I ever found some bare metal or rust I'd remove the rust and definitely use some 2 PART epoxy primer on the area.
I wouldnt use any type of rust convertor because that stuff usually plays havoc with subsequent adhesion.
Even spi epoxy does recommend Ospho brand rust convertor but its not to be left on the surface and definitely not left to dry. Or there will be adhesion problems with all topcoats even epoxy. They say if you have to use Ospho, scrub the area with it but KEEP it WET. Then rinse off with mild soap and water. Then normal degrease and epoxy.
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Re: Replacing Back Window

Post by pila on Wed Apr 06, 2016 7:03 pm

I primed my window frames with self-etching primer. In fact I primed the hole body with it before using conventional primer. Since the whole body was sand blasted, it wasn't an issue of rust.

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Re: Replacing Back Window

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