I dug this out of my bits box and thought I'd post it here. It was an experiment I tried on a few models with different colours, quite a while ago now. I was just trying different things, seeing what effects I could get.
This was a brown model (Swedish tank), and I basically got a bucket of water, sprayed some yellow and green in the water, which automatically stayed on the surface, and then dipped the tank in the water.
The paint sticks to the surface of the tank. It's like marbelling with aerosols.
The paint is a bit stringy, but if you use a toothbrush or similar you can brush the bits into all the grooves, or just leave it dangling for a swampy look.