I primed in black, and then applied a full coat of Reaper Master Series 'Rainy Grey' (essentially the equivalent of Codex Grey). I then applied a coat of MSP Ghost White at about a 30 degree angle. It covered nicely, but didn't leave much grey behind at that angle. Just a bit under the bottom lip of the helmets. I then applied Pure White at about a 60 degree angle.
I had a really tough time telling whether the paint was covering properly due to the lack of contrast between the Ghost White and the Pure White. Ghost White is a slightly blue-tinted white, but it's not starkly blue against pure white. So, I kept spraying pure white over and over. I kept seeing small grey splotches under the white, and in hindsight, i think that was because I'd used the same water to thin the white and grey as I'd used to thin primer and olive green. Dummy mistake!
Here's how things turned out:
The completed model is my only existing Sternguard model, and I painted him quite a while ago. I think he was the first model I ever painted with white parts, at least in my Ultramarine army. I'd painted white on other types of models in the past.
You'll probably notice that I used blue as a shade color for the white. It's a super-thinned coat of MSP Ultramarine Highlight, the same color as his armor. It provides a nice bit of contrast and delineation of the white armor parts.
As for the color difference between the two sprayed helmets, I blame that on my lighting. I think my lights were angled a little too far to the left, and that right-side helmet got left partially in shadow, or at least not in the direct lighting path.
I'm not sure how I feel about the new method. On one hand, the white is nice and smooth. On the other, it lacks any real contrast. While the completed, old model isn't perfect, it at least has definition. However, the delineation between the white and blue is a little too stark, and it ends up looking a bit cartoony.
One big hurdle I'm facing is applying white to other parts of the models. My Terminators all have white chest eagles, and I was thinking of doing the same for my Sternguard. I'm not sure details like aquila wings and Crux Terminatus skulls will look right without that deep contrast.
I feel I have a handful of options going forward:
1) Use the helmets as they are. Maybe they'll perk up a little when I paint the targeters, grilles, and lenses.
2) Apply my existing blue glaze method to the helmets as they are, then respray with Pure White. This might help the model more closely match my old style.
3) Apply my existing blue glaze method, and then reapply Pure White with a brush for better control and to make sure I don't "erase" the blue with the sprayed white.
4) Strip these heads, start over, and go for a black primer, then blue, then white. This might allow more blue to appear than simply glazing would.
5) Go back to my hand painting method, and only airbrush the blue armor parts of the model.
At this point I really have no idea which direction to go in.