Henry wrote: what colour should I paint the MGs it because i've note there in a black colour more than a metal.
For 1/72 scale I paint items like these black, then when dry, draw over them with a soft grade pencil - like a 2B or higher. Works fine for parts with plenty of detail but I wouldn't do it for large smooth surfaces - although maybe smudging it with a finger might work.
I just airbrush up to the blutac, if you angle the airbrush right you'll get a nice soft edge and it won't overspray but, I started out using clingfilm (or Gladwrap as you colonials call it ) or just strips of paper, tissues etc. It should only be slight overspray so it doesn't have to be too over the top, unless you're using rattle cans?