Some epoxies contain talc, so they're not 100% waterproof. So if urethane is supposed to go on epoxied bare metal, how is one addressing that issue? It's been stated you can't put urethane over finished topcoat. But topcoat is waterproof and many shops are installing glass over finished paint with no failures. Where's a glass tech with 2012 data on mandatory installation guideline documentation on new and repaired vehicles? I know that the existing urethane is supposed to be left in place after cutting out the old glass. That has not changed.
We'd get more information over Refinish Network than here. The ratio of pros and shop owners to hobbiests far exceeds that of this or a similar site.
Well, here is a certified "glass tech" and I said what has to happen, bare metal, epoxy primer, then urethane, PERIOD. Many components from the manufacture like lift gates will even have the pinch weld covered in tape and you paint the gate, THEN remove the tape exposing the factory primer (epoxy? I don't know, but factory oked primer) before you urethane the glass in on that primer.
In ICAR class it is made VERY clear, epoxy over bare metal. We here at the shop have a company called "Verifacts" come by with spot checks and this is one of the things they check, that there is NO PAINT in the pinch weld where the urethane bed is to be.
Yes "Epoxy primer" quality has a pretty wide range, and as far as I remember GM doesn't have a specific part# epoxy they sell. They use a term like " use a quality epoxy" or something like that, leaving the door open to cover their tush.
They have the same recommendations for bonding fiberglass body structural pieces to the aluminum frame on a C5 Vette, epoxy primer over the aluminum and then the GM "urethane" (not sure of the actual material description) over that epoxy to bond the panel on.
"Water proof" has little to do it, it's all about the tensile strength (I guess that is what you would call it) my book on the subject from class is at home, I'll look at it tonight.
But anyway, the bond between the basecoat and clear coat is one of the issues, think about it. If you put a 1K single basecoat then clear over it, just how good is that bond? How about waterborne basecoat? You are relying on the integrity of waterborne basecoat to hold in a structural piece of the body? HUH? I don't care what urethane, butyl, elmers glue, if you put it on top of a waterborne base/clear, your bond is only as strong as the waterborne base/clear! THAT is the issue.
Urethaning a window in over the bc/cc is a BIG no-no and not one bit better than using Butyl. Don't be so down on butyl if you are advocating urethaning over paint!
And coolblue, the one problem with the OEM manual is that it is going to give part numbers for products that aren't available. I have a manual but forgot to check it after I read your post, I will tonight. Also, cutting the glass off the urethane and bonding the glass back to FRESH CUT urethane IS the number one recommendation in the industry. And that is exactly how it is done every day. We wouldn't expect a glass shop to shop to strip out all the urethane and primer and repriming it right? Well slicing the urethane bed in half and putting a bead of urethane over it then setting the glass is EXACTLY what is done and is the recommendation in the industry.