Do you mean to use a strip of butyl tape on the pinch weld all around the windshield before installing the gasket and windshield, or do you cram it into the channel on the gasket?
No not technically crammed in there BUT where the gasket meets the pinch weld. OK after you have gasket installed on windshield or back glass run a rope inside the gasket to pinch weld area start at the BOTTOM and ending at BOTTOM of the windshield or back glass. Leave extra so you have a place to pull the rope out going left and right. Tape the extra rope to the center of the glass so it will not get in your way once you flip the glass over to go into the vehicle. Then starting at the BOTTOM of the glass/windshield run butyl tape starting at the BOTTOM around to perimeter and end at the BOTTOM of the glass (like you did the rope) in the area previously mentioned. With a high quality glass cleaner in a spray can (not Windex since it does not have the slickness as a good DOW glass cleaner has) spray the gasket area very good then spray the pinch weld of the vehicle. SOAK IT GOOD! This will allow movement of the rope and buytl tape until it drys. The high quality cleaner I'm speaking of foams as it is applied.... you more than likely know what I'm talking about. It really takes three people to it professionally (make life easier) without trouble. After laying the glass in the "hole" and making sure it is centered have a person on each side apply steady pressure to the glass. Do not allow them to let up at any time. With one person inside vehicle pull the rope out going left and right at the bottom of the glass making sure the gasket is over lapping the pinch weld. You might need picking tool to pull gasket over the pinch weld inside vehicle. After you have established that the gasket is over the pinch weld at the bottom proceed to pull the rope up the A-pillar on both sides to the top of the pinch weld. At this point the out side people can "pat" the glass in around the edges USING A CUPPED HAND. Using a flat hand will increase the possibility of breaking the glass... especially the laminated windshield. The tempered back glass is less forgiving about breaking. Basically you will have to "pat" in the top portion all the away across as well as have the people on the outside pulling down "seating" glass if it seems that it will not go in. The rope should be pulled inward to the center of the glass from each side until the rope comes out. At this point you should be ok and there you have it. Before locking the "self locking" gasket go around UNDER the gasket all the around the perimeter of the glass on the outside with 418 Dow urethane. The tip of the caulk tube should be under the gasket at all times until you have made a complete circle of the glass. A little bit will go a long way but if you use to much all you are doing is increasing clean up time. Lock the gasket in place and then using Adhesive cleaner (get in at NAPA auto parts)clean up the mess. You will more than likely go through alot of paper towels. If you get urethane on the car's paint the adhesive cleaner will not affect the paint unless its in primer stage and then it shouldn't be to big of problem. Let it set and dry up as much as possible before driving the vehicle.... 12 hrs should be fine. Now you want to use bed a glaze along where the gasket meets the body of the (the area that is under and will be hid with the reveal molding). This will eliminate ANY potential of air leaks or water leaks but I seriously doubt you will have that. Once you have ran a bead there you can smooth it out with your finger and then snap your reveal moldings on. You will want to where plastic type gloves just about the entire time. That's it... you now have a professionally installed back glass or wind shield.
If not tape, is it butyl from a caulking gun? If so, do you just apply it into the channel on the gasket before installing?
I believe I have answered that question above.... I hope so anyways. lol
Let me know how it goes. We have done numerous installs like this on numerous different older model cars and have yet have a return of water leaks or air leaks.
Feel free to ask any questions that I might not have covered.
