The gaps are irregular around window frame, but the widest are no than half inch (1.3cm) and the frame is flexible metal because the window is uPVC. I can use backer foam that's half inch wide and since gap is deep I can stack them on top each other for tight fit. I haven't used spray urethane foam. Which is better? I also have rope caulk, it's like putty.
Also how does one apply silicone for such big gaps? I have silicone but also the synthetic stuff like Lexel which allows you to reapply without removing old caulk