Hi Jo,
Thanks for the compliment! I mostly use Games Workshop / Citadel acrylics, making a suitable muck colour our of chardon granite (dark grey), graveyard earth (light brown) and black, which I slop all over the underframe. This is also thinly washed over the bodysides.
I then use a sponge to dab on a dark rust colour mixed to taste very lightly, which simulates chipping of paintwork - a technique stolen from military modellers! I then sharpen a matchstick and pick away at the rust spots to make sure none are too big, especially on the handrail type bits which tend to weather differently. A quick dust of Mig powders, a rub of pencil lead over the springs and buffer ends and that's about it.
There is a fair big of trial and error involved, and a lot of it is about removal rather than application but I hope that helps. I'm off to a toyfair tomorrow on the lookout for a couple more dogfish and / or seacows so I will try and get some photo of the techniques if it helps, depending what the weather is like when I do them!
Cheers, Steve