I use a synthetic brush for priming, as you\'re jabbing into crevices and the primer itself is probably not something you want up in the ferrule of a good brush (although I\'ve used sables to spot prime with no ill effect that I\'ve noticed). You can use a largeish brush, as you\'re painting the whole thing rather than needing to paint precisely. I\'ve even used a flat brush to prime. One thing I like about brush priming is that I\'m looking over the mini again, and if I spot a problem like a pinhole or missed mouldline, I can fix it and then go back to priming.
I thin similar to a base coat. The first coat, it\'ll have a grayish undertone. (I generally prime white.) Then the second coat will look fairly smoothed out. I do try to paint it on in sections using natural \'breaks\' on the model - so like paint the top of the front of a shirt to the belt, then rinse the brush and reload with primer, then do the bottom half under the belt sort of thing. I\'m not being super precise or careful, though. I\'ll often have the brush fairly loaded and then spread it out through that area. If primer really starts pooling in deep texture areas, I\'ll wick it out with a clean brush, but generally it\'s not a problem.