I'm assuming you are drybrushing your necrons in metallics over a black primer, am I correct?
Since you are knowledgeable about drybrushing, I assume you are a beginner/intermediate level painter. Here are some easy ways of doing rust.
Drybrushing
- Do a 1st drybrush of a heavy rust color over your black primer. Scorched Brown or Dark Flesh work great, and are better than Tin Bitz as rust dulls down metal (llok at rusted areas on construction or road works vehicles, forinstance).
- Follow with a lighter drybrush of Bestial Brown to represent lighter rust.
- Wash of diluted Devlan Mud
- Drybrush of base metallics (for instance, Boltgun Metal). Be careful not to go over the whole model: you want some of the rust to show through on raised areas too
- drybrush of a lighter metallic color to hightlight the bare metal.
- Wash of diluted Devlan Mud
Sponge weathering
- start with your standard necron painting
- take some blister foam, tear it into a kind of a stamp, then dab it into Bestial Brown. Wipe most of the paint, as if you were about to do a heavy drybrush. Dab lightly and repeatedly areas you want to rust: the random tears of the foam will create of very realistic stippling pattern.
- Take a small round brush and fill some of the rusted spots wirth Scorched Brown to create a darker, deeper rust.
- Wash the whole model with diluted Devlan Mud.
Sponge weathering is very similar to stippling. I did not add any highlight step to the sponge weathering rust, as it can create a confusion between lighter rust and hightlight. You can start adding that step when you have a better understanding of rust effects, and how you want to represent it on your models.
Pigments
Buy some rust pigments (look at Forge World or Mig Productions for companies that produce such things).
- Apply them with an old brush on areas you want to rust, it's OK to place small lumps of pigments. They look quite stark, don't worry as the next stage will dull them down.
- Let dry spirit flow over areas where you applied pigments, without touching the pigments as far as possible. Let it dry (1 hour time minimum, it's better to let it dry overnight if possible) to semi-fix the pigment
- Take an old small flat brush, or do downward streaking motions to simulate rust streaks, and to brush away the excess pigment.
- Seal with a layer of matt varnish (2 might coats are better), which will fix permanently pigments and dull them again.
The pigment method is really easy to do, and does wonder if you are painting in assembly line. Don't be afraid to try it. Be sure to use separate brushes for pigments and white spirit, to avoid messing your regular brushes.
Once you get confortable with those methods, you can combine them for extra detailed rust effects. Sponge weathering and pigments work great together. It also allows you to easily highlight the sponge weathered effect, as you will add another layer of rust with pigments.
Don't be afraid to experiment, and show us the results of your trials!