Drybrushing is a great technique in some cases, as stated earlier the dryer the brush the better the result. I use it almost only when painting metal. If you are patient and use a
dry brush- like when you think you'll have to get new paint dry- that has rather stiff bristles(?spelling?)-hairs you are just about right to start the brushing.
If you do this with boltgun metal on a black area you could get a surface that will have some "mirroring" (is that a word) qualities.
When I get my hands on that digicam again I'll post some pics on my tanks
