Here in my technique for instant rust.
I like to use Earth Safe Finishes Gel Medium because it has no Volatile Organic Compounds (VOC). You can read more about VOC on this page:
2. Sprinkle on iron filings to cover the gel medium.
3. Get the excess filings back into the container but funneling them in with a folded piece of paper.
4. When the gel medium is completely dry, (I like to wait until the next day), spray it with straight vinegar. Keep it moist by respraying it.
5. With in 3-4 hours the iron filings will turn the color of rust. (They are gray to start with.) The vinegar will make an iron liquor that will dye the background rust color.
The shirt above and the ginkgo leaves below were made with stencils from The Crafters Workshop. The shirt was made using a stencil called Capricious, one of their 12" x 12" stencils.