I finally ordered the rust effect paint I talked about in my Plaguebearers post and it came this morning :D So naturally the first thing I did was use it and my God is that stuff amazing! It has completely changed the look of them from a bit gross to being total minions of pestilence.
The effect is absolutely amazing and it's easy to use. The paint itself is Modelmates Rust Effect and each layer changes the depth of the rust. Now it says that it's a dye but I'd go further and say its basically liquid rust because when it dries it looks (and even feels) like actual rust which I think is actually amazing.
As ever, pictures are below. I took pictures of the standard bearer as it has the largest rusty surface area so you can see the effect much better than just on the swords.