The first step was to spray the model with a black primer spray. Once this had dried I drybrushed boltgun metal onto the areas where I wanted the metal to show through.
Next I painted brass (Coat d'arms Brass, which has the same formula as the now-OOP Citadel Brazen Brass) over the edges of the armour sections. You normally need two coats to get a good finish.
After this I painted the armour sections with Citadel Scab Red, with a tiny amount of Deneb Stone mixed in. The Deneb Stone makes the red much more opaque so that you don't need multiple coats to get good coverage. I also painted the bass with Codex Grey.
Then I gave the whole model a wash with Devlan Mud. Once it had dried I drybrushed the base with Deneb Stone, and that's pretty much it!
I will admit that the old GW Paladins are neither the prettiest or the most detailed models, so I am not going to put too much effort into them. The newer Chevaliers and the specialist units (the Errants, Castellans and Crusaders) and the Baron will probably get a more detailed paint job.
Update: thanks to some feedback from some of the chaps on the http://www.tacticalwargames.net/taccmd/viewtopic.php?f=7&t=22606&p=437112">Tactical Command forums, I've revised the design to include hazard stripes on one of the shoulders, and a decal on the one of the legs. I am going to get some Adeptus Mechanicus transfers printed to use on the rest of the models. Here's the latest paint scheme:
