I know what you mean. I have trouble often detailing with silver or gold without adding significant volume.
For example, doing edge highlights on a bolter used to mean that I had to choose between a line that was too fat for my liking, or that added a ridge. I was so unhappy with the results that I went out and bought every metallic paint in every major brand to experiment with.
Here's what I do now: To get a perfectly flat, thin metallic silver, I put on a thin layer of black (it doesn't have to achieve opacity), and then I paint either
GW Leadbelcher or VMC Gunmetal Grey using a FLAT brush. Both are thinned with water, but not a ton. The first coat of Leadbelcher always goes on nice and level, and I use Gunmetal Grey owhen I want it to come out very dark .
As Gunzhard mentioned, keep the brush clean. Then I use Nuhln Oil (which adds no significant volume) and I do detailing with Runefang Silver. After quite a bit of experimentation, I have decided that Runefang is the thinnest out-of-the-bottle paint. I have tried using VMA metallic paints from a brush too. They are very liquidy, but seem to add unwanted volume (ridges) to my work.
With gold, I either use a two-step or multistep process. My easy gold is to paint brown to opaque, and then use VMC Gold on top, leaving the recesses. The results are actually quite good if you can feather towards the recess with the tip of the brush in a single stroke, because the belly of the brush carries more paint than the point, and a slightly stippled effect of a metallic gold against brown looks like a fade-to-dark. I use no wash, and highlight the edges by going over them again with gold, or, if I really need, silver.
My multistep gold process is the
GW method, slightly modified. All paint is thinned with water.
1. Brown to opaque
2. 1 thin layer of Balthazar Gold, leaving the recesses brown.
3. 1 thin layer of Gehenna Gold directly on top of Blathazar.
4. Serraphim Sepia Wash
5. Thinned Auric Gold layer, avoiding recesses and allowing shadows
6. Thinned Auric Gold at edges
7. Runefang Steel for final highlight
I quickly did a shoulder marking on one of the Space Hulk termies to show you what I mean (it's the model I'm working on now, anyhow). It's not perfect, but you can see that even after many layers of metallic paint, it's still quite untextured and smoothly blended.