Scrape out old butyl tape or silicone. The frame must be spotless.
You’ve seen it before: that ugly white fog sitting between the two panes of glass in your living room window. You wipe the inside, you wipe the outside, but the haze won't go away. repair double pane windows
Look for plastic strips holding the glass in place. Pry these out carefully with a putty knife. Label which side is up. Scrape out old butyl tape or silicone