Here's an very easy tip on how I make really worn out and rusty objects. I this case we have an barrel/oildrum from the GW Battlefield Accessories Set
It is first primed black and then painted with Warpaints Gun Metal, leave some black in the recesses.
Then I applied a lot of Kromlech's Orange Rust (KRMA006) Weathering Powder, I applied it together with Citadel Lahmian Medium. I focused around the recesses and where rust usually are.
I then dry brush with Citadel Necron Compound.
Sorry for this last pic, it got to bright for some reason so don't really show how the final result really looks. But the last step is to wash the whole thing with Citadel Typhus Corrosion (A Technical paint that really has progressed into being one of my favorite paints as of late), I wiped some of it off in the middle to get a more uneven look.
That is all, really easy to do and you can do other variants as well with adding more Typhus Corrosion or another kind of wash. You can also do another highlight if you want but I went with this fairly simple method and I hope some of you might find it useful.