The curse of being 'good' at jigsaws

My heart sank when I saw the email - 'Jigsaw - help!!'. I had been contacted by a friend for some advice on finishing off some rather tricky sky. I sent across some ideas for sorting pieces into type etc and that seemed to be gratefully received. A few days later and no joy had been had - would I be willing to take a look? I won't go into details, but will say, that by the end of 5 hours (yes you heard me) that awful sky was done....I have nothing else to add....and never want to see this puzzle again!

p.s. I could tell you what made it hard....cut, colour...but don't want to think about it again!