Ok have done the mini tutorial you asked for. It would be better if i new the fuck how to use a camera properly, the the right lighting etc, but hey we can't all be perfect.
Anyways here is an easy 6 coat way to do white armour.
Step 1. After priming the model black paint all the armour codex grey. Easy. Remember as always to keep the paint to an even consistency, to thick and it will blob. Thin and it will pool like ink the cracks.
Step 2. Next go over the codex grey with Astronomica Grey (foundation paint) again keep the consistency smooth and slightly thin. Make sure to leave the deeper recesses codex grey.
Step 3/4/5. Now go clean yor water pot, clean your brushes with soap and clean your pallet. Reason is when you use white the smallest ammount of another colour will drity the white. Take again a nice smoth, slightly thinned coat and go over the Astronomica. Again leaving the deeper/shaded areas as before. All you do is repeat this step probably 2 more times until you have a nice smooth white finish.
What you'll be left with is a white armoured marine with sublte grey shading. Voila.
This particular model only took me 1hr and thats including taking photo's and writing this.