A customer of ours picked these two pieces up cheap on Ebay and wanted us to spiff up the appearance of the bastion and paint the tank. Here's the results:
The bastion was painted either with some very rough grey auto primer or perhaps that GW texture paint they came out with a while back. I mulled over what to do with it and came to the conclusion it would be quicker and easier to re-prime the piece and start over.
I primed it black and used various metals to bring the color back up. This would match the other Grey Knights items he has. Then I picked out some details with some gold and gave them a wash. I think it came out a bit better than expected.
I started by airbrushing the whole tank with bolt-gun metal from GW over a black primer. I took my time, working from the center of the panels to the edges, making sure there was more paint in the center of the panels and surfaces to give them the start of a highlight.