When it comes to painting big models like this Chaos Space Marine Forgefiend I like to paint an entire area before moving on to the next area. For instance I've painted the head and neck first to completion. Then I'll paint the body, smoke stacks and tail(s). After that I'll paint legs at the same time before finishing off by painting both arms. I learnt to do this after painting my first Tyranid Hierophant all at once, it was a nightmare. My second one was much easier to paint by concentrating on a smaller 'bitesize' areas one at a time. I'm going to use the same approach for my Tesseract Vault, I'll treat each corner as a separate model in effect. it will take longer but my philosophy is these models are a luxury purchase and I want to enjoy every minute painting them to the very best of my ability, so why rush?