Butyl was never used on these cars from the factory. Cruise night GM engineers without a degree pass that misinformation to other hobbiests.
Without name calling, I followed a respected member, Brain Martin's (MARTINSR) advise.
“Basics of Basics” Window setting
By Brian Martin
First let me say that the information in these instructions was compiled after years of installing and repairing rusty and leaking window pinch welds. I feel that with what I have seen in failed, overly rusty window pinch welds, and where I have seen pinch welds that have remained in good condition gave me a good idea on what works and doesn’t work. The rust found in window pinch welds is many times directly related to how a window was previously set. These instructions I feel give the best seal and long lasting results. Let me say that there are a few different ideas on things, these have worked very well for me. This instruction is starting after any rust repair, primer/paint. That would require MUCH more instructions.
First off get some 3M (or there are other brands) "pinch weld primer." It comes in a little 1/4 pint or so sized can and has a brush attached to the underside of the lid. Remember that you only apply this over a very clean surface. You should have a clean painted or primed pinch weld. The “pinch weld” is the area where the window butyl tape actually sits. If you will notice that is where you will see the spot welds that hold the inner and outer pieces of that area together. These pieces are STRSW (Squeeze Type Resistance Spot Welder) welded and “pinched” together by the welder, hints the name “Pinch weld”.
Before you install the clips be sure they are correct!!! You should start with either new ones or VERY nice old ones. When they are installed the top of the clip should be just a hair below the top of the pinch weld or body surface. I also recommend that you trial fit a piece of the molding to see if it fits correctly. This is time very well spent. If you find later that the clips are wrong, you will REALLY wish you did this trial fit.
You could even set the window in on some rubber blocks and install the moldings to be sure it will all work well.
The Butyl tape will not remain the advertised size when the window smashes it down. So don’t think that the ¼” butyl tape will STAY ¼”. That is why the tape you use is larger than the actual space you will have between the bottom of the glass and the top of the pinch weld surface. ¼” Butyl tape (3M is one brand, there are others) will end up at about 3/16, 5/32 or even down to 1/8”. The 5/16” will end up at about ¼ or 7/32”. The 3/8” will be 5/16 or 9/32”. The butyl tape can be flattened more, if you get it warm enough. So these numbers are used as a guide only.
After you install the clips brush the pinch weld primer on the bottom of the pinch weld where the butyl tape is going to lay. Brush up around the clips good. The paint gets scratched when you slip the clips on to the studs and this primer will help protect it.
Next you want to set the window in the hole and find what rubber setting blocks you want to use. There are a few that come with the butyl tape kit and you set them different ways to make them higher or lower. Be sure that the window fits evenly all the way around. Run a piece of masking tape from on the glass out onto the body on the sides and the center of the top and bottom. Really only one should be needed but it helps to have more so you helper can see one well too. These tapes are then cut on the edge of the glass so you can remove the window again. Just leave the tape on the window and the body for alignment purposes.
Be sure the window is clean and free of residue of any kind. With it lying on its back, put the butyl tape on the edge of the glass without hanging off. Roll it out and as you go you can pull the paper off it so that way when you come to a corner you can get around it. If the paper is on, you can't get around the corner. But you want the paper on as you are going down the tops and sides to help you get it straight and so you don't touch it. At the point where they meet, run one next to the other and at the point they pass, cut it with a NEW razor blade. Then DON'T touch the tape, using the razor push it, blending it together. Most guys will tell you to make this joint at a certain spot like on the side or at the bottom. I really don’t think it matters as far as leaking, if it is done incorrectly it WILL leak, what does it matter where it leaks? The point is, do it properly and it won’t leak. I like to look at where it will be the least seen. On the sides it is usually seen and looks like hell. I usually make this splice at the bottom; it seems to be the most hidden spot. You can get the butyl tape in ¼”, 5/16” and 3/8” in diameter. I have found that the 5/16” is the most common with ¼” being used in late model cars on the quarter windows. The 3/8” is rarely used and can really get you in trouble. If you set the glass with the 3/8” you may find that it is impossible to install the moldings because the glass is too high!
NOTE! Some of the new (and replacements for older cars) windshields are thinner glass and the 3/8" butyl tape is needed to set the glass. It raises the glass up to make up for this. So, check with a good trial fit first! If you have an original glass DO NOT use 3/8” butyl tape it will raise the window up too high and the moldings will not fit!
Then with your helper set the glass in using the masking tape as a guide. DON'T LET IT TOUCH until you have it where you want it. YOU WON'T HAVE A SECOND CHANCE the butyl tape sticks RIGHT NOW and will not let go. If you have the glass a little over or up or down too much you can move it a LITTLE after the glass is lying in. Before you push on the top of the glass, using a plastic setting tool or even a piece of wood you can wedge it into the pinch weld channel and pry the window over, VERY carefully. At this time it is best to get it warm. If you can get it in the sun and warm up the glass and butyl tape and push on the top of the glass to get a good seal all the way around.
Now comes the real important part, sealing. First let me say that I have done many windows and used NO sealer what so ever. The Butyl tape alone should be sealing enough. But as an added "safety net" I do the following.
I take a plastic "bondo" spreader and cut it to about a half inch in width. I also round the corners with the razor nice and clean. Using a high quality urethane sealant I put a little strip right on the side of the glass edge or on the butyl tape it's self. Using the spreader I spread it down the side of the glass over the butyl tape to the bottom of the pinch weld, forming a seal from the top to the bottom vertically. In other words this sealer is covering the edge of the glass and the butyl tape, THAT IS IT. The sealer goes down to hit the bottom of the pinch weld right where the butyl tape is resting, but that is it. That bottom edge of the sealer where it hits the pinch weld is most critical. Be sure that it IS sealing down at the pinch weld. It should have a nice clean line as it hits the pinch weld. If it pulls up off the pinch weld, it is not a seal. Don’t assume because you have that sealer down in there, that it is sealed. If you put the bead of sealer in there and then spread it with the bondo spreader with pressure against the edge of the glass and butyl tape, it WILL flow down and seal the bottom, you just have to be sure you are doing that. DO NOT PUT THE SEALER ALL OVER THE CLIPS OR FILL THE CHANNEL this is a DEATH sentence to you job. When you are done, you should be able to still remove the clips if you wanted, that is doing the job right, no sloppy sealer all over the place just because the molding will cover it. Every little nook and cranny created by the sloppy sealer is a place where water will gather and sit longer than needed. This is a place for rust to start.
If you do this and you and see the seal has been made from the top edge of the glass down to the bottom, you know it will never leak. When water gets in there, it will be able to evaporate. If you fill the channel with sealant, the water gets in and can't get out!
You now can put the moldings on and off at your leisure. If your molding is giving you trouble and not clicking into place on the clip. You can take a rubber squeegee and place it on top of the molding and then a small block of wood and tap; JUST tap lightly on top and you can get even the most stubborn molding on. But if you trial fit them and didn't put sealant all over them this shouldn't be needed.
Note: These suggestions are for full frame cars, vintage cars. Most every late model car was designed to have the stationary glass “urethane” set, DO NOT use butyl tape on a uni-body late model car. The glass is a part of the structural design of the car. Using butyl tape could jeopardize the structural integrity of the safety features in the vehicle. You can in fact use the superior urethane setting technique on any car if you choose. However, it makes things a lot harder. A late model cars window seal can’t be seen. So a sloppy urethane job is hidden. On a 68 Camaro for instance, you SEE the windshield sealant from the inside of the car. So to use a urethane needs a real pro to apply. Applying a bead of urethane that is perfect all the way around the window is next to impossible, most pros don’t even get close. They don’t have to get good at it, 99.9 % of the cars they are doing are late model where it is completely hidden. So you would need to use a foam tape “dam” along the sides and bottom of the glass at the very least. This “dam” holds the urethane from “oozing” out into the passenger compartment.