The guide for rust in the follwing threads will only really work on light to medium colours, either in Enamel or Acrylic based paints. The rust won't show up on very dark colours, black being the worst and least effective base colour to use! The most effective colour is white, however, for best effect it's best to stick to either Satin or Matt. It's very important to allow the base colours to dry thoroughly before applying any rust, as all it will do once the white spirit hits will be to mix the base colour in with the rust and you'll end up having to repaint that area of your model again!!!
Firstly, decide as to the degree of weathering your going to have. My advice is, less is nearly always best! The most common mistake (regardless of experience) is to over do the weathering treatment. It's very difficult deciding when enough is enough, and it's hard work having to correct things afterwards if all goes wrong.
Right, that's my recommendation bit all done with, let's get on with the first stage:
*Start by finding a piece of plasti-card, roughly 5" or 6" inches square. Spray a light coat of plastic primer, white if you have some as that will save some time!
*Allow the paint to dry off, an hour should be long enough assuming your using Acrylic.
*Squeeze out a pea sized amount of "Burnt Sienna" into a mixing dish, and add some White Spirit.
(the viscosity we're looking for would be "Medium") You don't want it too watery as we want it to sit happily on our test piece without spreading out in all dirrections!
*Once you've had a little play with getting the viscosity just right then, using your fine brush apply some Burnt Sienna to the top area of your painted plasti-card.
*Allow this to sit and settle for at least 30-45min's at room temprature. You'll know when it's semi-dry once it turns dull in tone.