I have never painted a white army before and had some terrible experiences using white paint as a base coat in the past - my main problem with using white paint as a base coat is I am color blind and can't see any of the model's details after the model is painted so I really struggle to paint it after it is based. I recently tried a test mini with Armory white in a spray can over a thin black coat (this way I could still see the detail)... Unfortunately this spray is rather heavy and coats over a lot of the raised details on the mini.
Next I decided to talk to some members of my club who are great painters - Dave (Crazy Dave), Aaron (Swags) and The Paul (TPM of course). They all had some great suggestions and I decided to combine their techniques for painting white. It is very simple but very time consuming since I paint everything with a brush straight out of the pot. That's not a big deal to me though as painting is simply for the enjoyment of the hobby. Something nice about white armor with blue trim (e.g., World Eaters) is it's a great combination on the color wheel - they work very well together.
I start with a soft blue grey base coat (the old GW Space Wolf Grey comes to mind). The blue grey base doesn't need to be completely covered by the next layer (light grey) so you have an excellent shadow in the recesses that works well with the blue trim and you can even keep some areas exposed that will be blue... A bright blue over white will really pop though. Like I said the next layer is a very light grey... Just paint everything you want to end up being white in the end.
It used to that white paint was very thick and you had to mix it with a relaxant to thin it down... To me that was a major pain and another one of my big deterrents to painting white. GW has some excellent whites now such as White Scar White which is very smooth, goes on easy and doesn't need much thinning at all.
So basically there are three main layers:
1) Blue grey base coat
2) Light grey layer
3) White final layer
Once you are done you can shade the white around edges with various levels of grey to create more shadow effects and some contrast plus grey is very easy to shade as there are many good greys to choose from in the market.
I am using a blue wash for greaves and armored straps as it matches well with the blue.
White Armor versus Other Things like Cloth and Skulls
The white armor will look cold since its applied over light grey. I want a contrast for other things such as cloth (e.g., purity seals) and skulls. If you're painting GW miniatures there's always going to be lots and lots of skulls.. That's just the way it is... Heh ! I want a contrast between the armor because otherwise you'll end up with a model that looks a lot like a black and white photo. I suppose that could be cool but it's not the visual image I want... So I want a warm appearance for things such as skulls. To achieve this effect go back and paint brown over the skulls and other things such as purity seals. Start off with a darker brown and then shade up a couple layers with lighter browns. This is really quick as your only painting little things, not the entire miniature. Finally apply the white.
You'll see what I mean when you're done... The armor looks cold while the skulls are warm in appearance and it makes for a very nice overall appearance.