The way that I handle white is to always tint it with a little bit of other colors, eg yellow or grey. Never use white alone except for the final highlight. By adding a little (make sure it is very very little), you will be surprised that all you need may be just two coats.
Check out the Cont. Marine thread in Joe Hudson's Vbench, he painted that figure in 3 different whites and gave the mixes so check it out and you will have a great start!