Chopped out the rust on the inside of the screen frame last night and even managed to find some good metal to leave in the middle as a reference 
A small amount of the outside of the frame will need chopping out too, but I’m planning to get the inside done first so I can then use that as a new datum. As a reminder to me, (as much as anything else), don’t forget to drill holes in the new frame pieces first to plug weld onto the old frame later! 