So this is the Gundam tutorial for *busy people* - you can tell by the way I spray the pieces. The plastic frame which holds the pieces is called the Runner - the bit of the runner which attaches directly to the piece is called the Gate.
What many people do is cut out all the pieces, sand down, go over with primer, sand down again and then paint - but that would mean that I would need a million years just to complete one model!
Not having a million years to spare, what I do is use a method which some people may call the "darn-right-lazy" method which is to spray the whole runner.
What I do then is to cut out the pieces as I build the model and then spray over any visible nip (snip?) marks. Nip marks are the marks left over in the piece after nipping out from the runner.