One big thing that you can do to speed up painting times and get better results over time is to "Unit paint" and by this I mean actually paint the whole squad at one time. Since you asked about Necrons, I will detail this out on a typical Necron Warrior Squad of 10 guys, and since you asked I will tell you how to get a Rusty look to their Metal parts.
Prep Takes about 1 hour although it should be spread out over a few sessions.
1.. Assemble and "Wash" the figures (use soap and water to remove all your dirty fingerprints and then rinse them well and let dry). It is often a good idea to do all of your Base work at this time on too, as the Spray will help to seal any sands you use.
2.. Spray them with Chaos Black (hardware store spray works too; but make sure its "Flat" and non-textured) also remember that 2-3 very light coats is always better than on heavy coat of Primer. When this is done, inspect your figures and use a "Tank Brush" and Chaos black paints (thinned a lot) to tough up andy areas you still missed.
3.. Take 10 old paint pots and some Blue tac and afix the squad to a pot; now you can hold onto the pot, while you paint the figure (easier on the hands and prevent touching the figure so much).
Painting Metal areas All of thee steps get doen to all 10 figures before you go onto the next step.
4.. OK, get a "Standard Brush" and lay down a coat of Tin Bitz to all of your metal areas.
5.. Keep using the standard brush and splatter/splotch some Gun Bolt Metal to all of your metal areas.
6.. using starndard brush add the primary colour to any other parts of the figure (lets say Dark Angles Green in this case).
7.. Use a Tank-brush to do a "Wash" of DA Green over the entire figure (a wash is a mix of paint and water that is so thin that you can actually read print right through the Wash).
8.. Do another Wash using Blood Red over the entire figure.
9.. Dry brush all the metal areas with Gun Bolt Metal, and done worry about touching some of the green as it wil get fixed later. Also make sure that this is a fairly good dry-brush so that 30-50% of the metal is actually brushed onto.
10.. Do a selctive dry-brush over all of your metal areas using Silver, now make sure that this is a much softer dry-brush so that may 10% of any area gets brushed onto.
Moving on to Primary colouring
11.. Now, using your Standard brush go over about 80% of your "Green" areas with a lighter shade of "Green" (for this case I will say Scale Green).
12.. use a Detail Brush and go over about 40% of your "Green" areas with a lighter shade (lets say 50/50 Scale and Goblin Green).
13.. Use your final highligth shade of "Green" (Lets say Goblin Green) and your detail brush to edge all of your "Green" areas.
Finishing the figure but at this point it is playable for sure.
14.. add a third colour for any icons, eyes, etc.... usually done with the detail and fine detail brushes. (If need the steps on this, ask)
15.. Do some dry-brushing on the base with to give it a Tone.... (If you need the steps on this, ask)
I know that sounds like it would take forever; but if you sat down for an hour or two at a time and did a few sessions you will find that in about 10hours of work you get 10 guys done, and they are actually well above "Table Top" ready. This is the method I use for all squads and feel free to check out my Website (klucas.piczo.com) in the gallery - there are a few good tutorials of different units being done. I hope that helps you out but here is a picture of one of my Destroyers (I didn't do the "Rust Look" and I did add a Green Ink to the overall armour to give it the ceramic look but all the rest is as per tutorial).