Not really my department. I would probably chisel out anything that didn't seem solid, render up to a thin piece of wood, take the wood out and fill with frame sealant. That might not be best practice though. Hopefully someone with a better understanding of the materials will come along.
Thanks - i'm fresh out of shrunken heads so stripping out and replacing sounds like the best course of action.
Any pro tips / considerations?
I was going to scrape away the blown stuff and chisel away a couple of inches of good stuff, re-render and seal around the windows then paint.