Found this little gem on papanurgle.net. An awesome article about producing really dirty looking white Plague Marines.
Here is a step by step guide to how I paint my plague marines. It’s pretty simple and i use the same technique on tanks, dreadnoughts etc. First up spray white and touch up with skull white paint, then, using the foundation paint set (which I water down a little to help the paint flow) paint all the metal areas orange, armour trim green and bolters red. With the red and orange I’m not very neat because I use such heavy washes that it covers up any wavey paint lines or missed bits etc. Finally paint the eyes shining gold: 
Now we come to the washes all of which are pretty heavy and straight from the pot. First up wash with black aiming for the deepest, darkest parts of the model AND all the coloured areas. 
Then brown covering the whole model 
Then green only covering the armour and trim 
Next we start on the stripey white. I have a pot of skull white that has a lot of water added to it. You want it to be thin enough that it flows easily (without pooling into drops like water does) and you can see the colour coming through the paint after one coat. Then it is simply painting little lines on the armour. I find that going section by section works best, going over the armour with one, two or three passes to get the effect I want. Basically it’s up to you and gets pretty obvious as you go. A good tip is to leave little ‘drip’ marks under things like studs by simply painting white around the ‘drip’. It looks cool. Here are two half way shots: 
Next up go over the green armour trim with the foundation green to cover up any mistakes with the stripey white and then catachan green in the corners or peaks as a highlight colour. I like this as its subtle and I want them looking dirty rather than shiney…so a ‘dull’ highlight works best I think. Now using the foundation orange again I go over the metal (washed orange) parts but this time i use a crappy brush and ‘stipple’ bright orange radomly. This looks pretty good, it’s fast and adds extra depth to the rust effect. On hoses etc be careful not to over do it. Then you can (i dont always, again because I’m after dull and dirty) use the foundation red to highlight the bolter. 
Once thats done the last thing is to paint on (NOT drybrush) some boltgun metal over the orange. This tip was given to me by Mabs and is a fab, fab tip. Think about what bits of the metal will be used the most and therefore have the rust worn off showing the bare metal underneath….like knife blades and where your marine will be holding his bolter..etc etc and then add some in random places so you dont have too uniform a look. 
The finished guy: 
You should go and read: