Here are the simple directions to do this technique.
1. Cover the object you want to "rust" with your water based glue. (Never try this with solvent based glues.) A pretty thick coat works better then a thin coat. It doesn't have to be even since it will bubble up anyways. I used only chipboard but see what else may work.
2. Drop 2 to 3 colors of alcohol ink onto the glue. It will react with the water in the glue and spread over all the surface.
3. Give it a few seconds to dry and then hit it with a heat gun. Keep the gun on it until it bubbles up. Be careful not to let the bubbles pop or the ink color will have holes in it that will reveal the chipboard color beneath.
4. You can keep drying it until it stops bubbling or if it is taking a long time let it dry a little on it's own. If any of the bubbles have not sunk back to the surface just pat them down with your fingers. (BE VERY CAREFUL NOT TO BURN YOURSELF!!)
Click on the picture to see a larger view and read the details of each sample.
These are the results of the various glues that I had available. (Earlier today I bought some Scotch clear glue and some Aleene's Clear Gel Tacky Glue to see if clear glues reacted differently.) I had the best overall results with Aleene's Tacky Glue but I think any kind of plain old white glue will work. The 2 that you should stay away from unless you want a look full of "holes" is Diamond Glaze and Mod Podge. I also tried using other brighter colors to see how they would look and I had mixed results. (The metallic alcohol inks didn't work at all.) If you leave the chipboard it's plain color it will dull the brighter inks, so I painted the chipboard white and it helped the colors pop out. I personally didn't like any of the other color combinations as much as the "rusty" ones.