I am new to the painting thing too. I only partially constructed my Space Marien Devastators before painting.
I left their arms and weapons off. This left me with the the legs, body, head and back pack to paint as one. This allowed access to their chest to paint. In the case of the heavy bolter it gave me access to his legs as well. With the weapons off I had easy access to the marine side of the weapon.
Its nothing something I looked up. I just felt it was best for me to allow myself as much access as possible.