I wouldn't have thought rubber would be the ideal tool, too soft. Couldn't indent necron plastic with that.
That highlighting thing with the hessian/web, yeah, you got it. The same amount of strokes to convey the texture each time, as you go up in highlighting stages,
That shadow vessel look like it could be equally served by the way of painting those pics Chrome posted. That is, either doing all the highlighting then freehanding the weblike cracks, or, freehanding the cracks with thin strokes of masking fluid before doing the highlighting stages.
Talking obsessive behaviour on a painting forum, you picked a good place for it.
Your idea regarding necron material, I dunno if that's canon but it sounds good to me. Convincing. The way you want to render it it's got me thinking of organic metal materials, the cracks reminiscent of veins, nanobot construction, or part of its unique oxidisation process.
Not wanting to repaint but just add another layer you're pretty much in the position of needing to freehand some sqwiggly lines on then eh. I think I'd go for black lines in the deepest areas, and start to move into tinbitz and bronze for higher areas of the webbiness. Though moving from black through the boltgun and chainmail metallics could work more well (more subtle) if those webby cracks maintain a progression just a shade darker than wherever they're going. That is, as the main body transitions from boltgun to chain to silver, the strokes should remain a paint, black to boltgun to chain.
That might end up a bit messy, all those layers of paint. I might prove prudent to render the cracks in layers of black ink, thinned with water to kick the pigment into being less dominating. Some very thin strokes on the upper layers, once dry that could be repeated further down to darken up the strokes that require it, and repeat as neccesary.
You thinking of doing it all over? As if it's a natural state of the metal. Or, thinking of just doing a bit of it here and there, as if it's a state of the metal being internally repaired. The latter could add a bit of variety amongst a unit, each having a differening amount of that webbiness, in different places.
If you have some spare bits around, parts, shields, or even bases, I'd experiment on them first. I got some Necrons half finished myself somewhere. I blame Dawn of War for making me want them. I might do something like this on them cos plain metal is isn't terribly exciting is it eh.
Looking forward to seeing what you settle on and how it comes out.
