I found a techniques which works really well for battle damage on ships that are supposed to be LARGE. Apply the paing on the area to be damages VERY heavy. Then cook it CAREFULLY with a candle flame. The paint will crisp, bubble and lift off from the surrounding plastic looking like burnt sheet metal on a ship of that scale. I discovered this when taking a compltetly screwed up Enterprise model and making it into the Constellation - I think every Trek modeler has a post plenet killer constellation around after messing up a "nice-n-purty" Enterprise attempt. After cripsing the paint I found that laying several (many) light coats of matte clear kept the burnt paint from flaking off. It really works well.
Jedi Dade