This has always been a headache for me: white colored paints have coverage issues. Most paint brands need to be diluted: with white it is very, very, VERY essential to get the dilution just right. If it is too dilute, it will be runny, if it is too thick it will splutter in the airbrush. (The reason for this post is the Amusing Hobby Ferdinand: ran out of Tamiya Flat White.)
Small areas can be done using a grey base (easier to paint), and then painting it over with Vallejo’s white primer, diluted somewhat, but not too much using a brush. It is much more forgiving than most other white paints as it looks extremely dense in pigments.
I have not managed to paint large white surfaces with hairy sticks (it is doable), yet, so it will be only airbrush here.
Larger areas- such as you can see on warhammer miniatures or tank interiors- however are a pain to paint. You are supposed to fog layers upon layers of white mist on the surface, with infinite patience, but let’s get real. I need something fast.
The best method, so far has been using Tamiya’s flat white. Normally I use a dark primer (pre-shading), and it is hell to cover with white. Not so if you use Tamiya. The method is simple: dilute it just a bit, so it stays thick but not too thick, add some retarder, crank up the pressure on the airbrush, and just have a go at it. You still have to be careful not to swamp the model with paint, so use lighter coats, but you can go back and forth without having to wait for the paint to dry. It works like poweder coating -the paint immediately dries as it hits the surface, so it will not have time to pool.
There is some experimentation needed, because if it is too thick it will splutter, but the method is a flexible, if a somewhat brutal approach to painting. With this you can cover larger areas effectively in one go. If you need off-white or ivory you then just mist it over with the appropriate color – much easier to create effective covering. (The same could work, I suspect, with flat white primer sprays, but I never had luck with those, either – the coverage was terrible, so it had to be applied in many layers, which caused paint buildup, and was not very easy to control the paint coming from a big spray can.)
That is it, basically. Thick Tamiya Flat White at high pressure – instant white surface.