First of all, you can do classic metal:

1) Boltgun metal
2) chainmail
3) mithril silver

But of course, there are many different ways of painting metal more advanced than this one.

1)boltgun metal
2)chainmail
3)brown ink, green ink, black ink mix. (you could use washes instead).

1)tin bitz
2)boltgun metal

1)boltgun metal
2)chainmail
3)wash some areas with watered down vermin brown to create a rust effect.

1)chainmail
2)devlan mud

1)boltgun metal
2)chainmail
3)brown ink

1)boltgun metal
2)chainmail
3)brown ink/flesh wash mix.

These are just a few ideas which i hope you find useful.