I can honestly say I haven't done rust on ships yet, but personally, I like the use of pastel chalks, and would submit they work just as well on a ship as they would on armor. I scrape a little of the chalk off with an X-acto knife onto a napkin, and then with a soft brush, pick up the chalk dust and begin spreading it where I want it. If it goes somewhere you don't like, a water-moistened q-tip will remove it. One point of advice: Don't get the oil pastel chalks...they don't work very well. I bought a set of earth tone chalks years ago, and am still using them--they'll last you a long time.
A second way to simulate rust is to dry-brush rust-colored paint where you want it to go. I like Testors MM enamels for this because I can get the right amounts and shadings in the right places better (more efficiently) than I can with acrylics.
One other method I've seen but never used is for simulating rust "runs" on ships (or other vertical surfaces). Take a 3 X 5 card, and cut a small slit in it the size of the run you want. Hold it about 1/2" or so away from the surface and spray rust through the slit with your airbrush....Instant run. Holding the card away from the surface will give the edge a little "feathered" effect, instead of a hard line that would result from direct masking.
You can also make your own real rust. Take a steel wool pad (about a 4-0 size) and put it in some salt water and leave it in the sun outside. The pad will oxidize over a few days to a week. Pour off the water (or let it evaporate), let the pad dry, and voila!...instant rust powder.
Places like www.micromark.com also makes a chemical I believe is called Rustall. I think it's a 2-part material that you brush on to your kit that actually makes real rust. I don't have a catalog in front of me, but I think the kit runs somewhere around $10.
Carnage, these are just a few ideas. There are many more out there. I would encourage you to try some or all of them, see which ones work and don't, then go with what you like. Good luck!
Oh, yeah, and Welcome to the forum family. Hope your questions get answered!
Hope this helps some.
Gip Winecoff