I would use spray for the exterior--much smoother application. For other things use a brush, but not one of the cheap plastic bristle ones. If you can, invest in a couple of nice brushes. Don't have any recommendations--been about 20 years or so since I did a model. If I remember correctly, brushes can get outrageously expensive. I don't know what kind of brush goes best with model paint.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose