I generally avoid dry-brushing unless I feel it's absolutely necessary. When feasible, I will thinly paint on the textured surfaces, because I really like my colors to "pop."
Recently, there have been times where I have had to dry-brush some metallics, and I have about every brand at my disposal. I find, generally, that Reaper Pro and Master series are FAR superior to other paints, but for metallics, I have tried everything to get a paint that will:
1. Adhere to the brush, and not spread metallic dust all over the miniature.
2. Adhere to the mini. I think that #2 goes hand-in-hand with #1.
I have tried different brushes. I have use everything Reaper and Vallejo has to offer, and even the GW stuff (whose primary accolade is "most unintuitive paint pot design," ever).
Fundamentally, my problem is that it's flaking (or basically, dusting the whole mini with silver/gold/copper flakes), or not adhering to the raised area.
I always use flow improvers for normal, non-metallics, and love highlighting/shading. For these metallics, I am not using anything at all, hoping that I can do whatever I can to make them remain viscous (the metallics seem fundamentally watery, anyway).
Any ideas or suggestions would be appreciated.