Hi Arthur, I have an article on weathering in Model Boats later on this year but in the meantime there are plenty of books available on the subject. One of my favourites is “The Art of Weathering” by Martyn Welch. I’m not sure if it is still in print but you can still get hold of copies from good book shops. I bought mine only a few months ago so they are still available. Unfortunately it is all related to railway modelling however most if not all the techniques can be adapted to boats and ships.
Very briefly as regards rust I would have a pallette such as an old white saucer and put a blob of orange, brown and purple on it. Dab your brush into the paint and creat a range of shades in the middle that every time you pick up some paint you adjust with a bit more of each colour. You should use a dry brushing technique in a vertical direction and build up a rust effect by streaking the paint downwards.
Some use oil paints for rust and there are powders and pastels but the subject is huge and you need to get hold of a good book such as the one above and start to experiment. An old hull or model is always great to play with and when you have covered it with ‘rust’ spray the whole thing grey and start again!
Have a look at this site where I used the above technique on this destroyer bow. I was only really playing at the time and I niow think there is too much but I did like the effect and went on to use it on other models:
There are also some useful tips in this thread: