Hand brushing white paint is the highest mortal sin in this hobby. Becuase it is the most translucent color/tint there is. No matter how many coats you put on, you will still see through it.
You can only acchive perfect white if you spray. That's it.
Best thing to do is to paint the model with white first, mask off the parts you want to keep white and paint the rest of the model.
Sorry for the bad news
Bernie the K.
St. Paul,MN