These are my exhuasts in my nearly complete HL Tiger overhaul... I was saving all the pics I've taken for 1 long build thread, but as you asked..

Outside of their covers (with the rest of the "accessories" )


Inside their covers (which are just painted plain yellow at this point)


These were done with nothing but tamiya acrylics. If you like them, here's how...
First off, I painted them all over in XF-16 Flat Aluminum to give a metalised undercoat (thinking about it, although I undercoated the rest of the tank in car primer, I didn't with these, I was too eager to get on and paint something!).
Then I gave them a thorough wash in 50% XF-64 Red/Brown, 50% thinners. Then it was a wash of 25% XF-1 Black, 75% thinners.
After those were totally dry, I started to dry brush with multiple layers of XF64 Red Brown mixed with more and more XF-57 Buff, making it lighter as I applied the layers.
If at this point it starts to get too light, another wash of very thin black will sort you right out.
Finally I gave it a very very light dry brushing of orange (this was in fact some old Citadel Miniatures (now called Games Workshop.. and if you remember CM that'll give you an idea of how old this paint is and it's still good!). I'm sure Tamiya's XF?? Orange would work perfectly well.
And that's it. I was REALLY pleased getting the effect I was after, which like you say gives that look of constantly heated pipes that have a solid, non-flakey layer of rust all over them.

In a week or so I'll post my whole build thread, of my HL Tiger 1 which has been fully "renovated"