Nothing looks more like rust, than, well, rust.

Back in the US I had a brilliant product: a two-part rust effect product. One flask had suspended iron particles, the other was an activator (an oxidizing reagent). You applied the metal suspension to the surface, and after it dried, you added the activator. After some days (or hours, depending on how much activator/iron you used) you got a very realistic rust effect. (It was rust, after all.) You could play around with different reagents to get different colors- after all any acid or hydrogen peroxide would work very well, and produce slightly different mixture of the different iron oxide variants. (Being in Florida, it was also a very viable option to simply leave the model out on the patio… the 100% humidity and high temperature was quite effective rusting anything, anyhow.)

The armor of my orc warrior was done using this method. (In this case the rust effect was used to depict a rotting leather armor; the same product was used on the ground as well to simulate dirt.)

Modern Masters and Porters Paints have similar products, but they cost an arm and leg.

However, you can make your own rust pigments, and quite easily at that.

Simply use scratch brite (metal, of course). Put some steel wool or scratch brite into a cup, add vinegar or peroxide (or any acid of your choice), and stick it somewhere safe, where nobody will disturb it. Let it stand for a month or two, and wait for the liquid to evaporate. (If by the time it evaporates parts of the iron are still unoxidized, simply refill the cup.) You can even experiment: depending on the strength and nature of the oxidizing reagent you use, you will get a slightly different colored mixture of rust particles.

The result will be a brown mess; if you break it up in a pestle you will have finely ground iron oxide, ready to be used. As the orc warrior has demonstrated the pigments have more uses than simply depict rust: they have a nice, brown range of colors which can be used in groundwork, deposits, etc. (Some of the rust I’ve made this was was used to simulate deposits collecting in the nooks and crevices of the T-62 wreck I’m building.)