One of the Youtube channels I watch had a guy changing a windshield. He said that the old butyl doesn't need to be completely removed, just cut back to good solid cured butyl.
I used butyl windshield sealant to set windows in a cab I built. It wasn't too bad, but you have to watch the glass doesn't move until the butyl sets well. I masked off the body to get a clean edge where the goo meets the paint.
I didn't mask the glass. After running a slightly too large bead of butyl, I used a stiff piece of plastic (one pass is best) to strike a joint - One end of the plastic rode on the glass while the other end hung well beyond the painted body. That leaves a deposit on the tape. The "joint" will have a nice smooth fillet. The glass will have a long ugly snake that's separated from the joint where the plastic left a clean line.
When the butyl cures, carefully remove the snake with a single edged razor scraper. In the pictture below, the masking tape was removed from the painted area and glass scraping is in progress. The blacker-looking irregular edge is the inside that hadn't been scraped at that point. That is what smooshed out on the back side of the glass when it was pressed in. It was removed by resting the side of the scraper against the strip of metal where the sealant was applied.
