I have followed the scheme I have set out in the Legion Medusa / Rhino guides (see them for details.
I painted the Demolisher Cannon with Leadbelcher on the main barrel then with a frayed brush I over brushed it with a semi dry Chainmail on it's raised sections to give it a brushed steel effect. The banding was painted with Warplock which was then raised to Brass.
I then washed the barrel with a coat of Agrax Earthshade followed with a light sponging of Runefang. Then drybrushed Abaddon Black around the barrel.
The exhaust vent tops were washed with Druchi Violet then drybrushed with Abaddon black.