The basic technique is this:
Airbrush the large surfaces
Add detail colours
*Highlight details (should have done this but did it later)
Spray with Future floor polish (Arcylic gloss)
Shade with thinned artist oils
Wait ~24 hours and wipe off as much of the oils as possible
Finish with dull coat spray.
I'm really happy with how the oils turned out. For large vehicles I'd say that this is the way to go. For minimal effort you really end up with a nice looking finish and the process is very forgiving. If you end up with too much you can wipe it off with a q-tip and a bit of mineral spirits. Not enough and you can do it again.
Oh, and yes I did drill out the multi-melta barrels after the comments from the previous post.