Instead of using glue, try some liquid mask. Or try the 'ol salt technique. It's a very easy way to accomplish this.
Sponge on your acrylics. A base of red oxide primer is usaullay best, because if you make a mistake that goes down to the primer, it's easy to fix up. Then spray down the body with a mist of water. Sprinkle the area that you want the rust to show though, with koscher (larger crystals) salt. Salt will stick to the water and stay on the paint when the water dries. After the part is dry, spray over it all with whatever top coat you are using. After that dries, knock off all the salt. After that a tooth brush and some 70% isopropyl alcohol (not 70 proof) will help you feather out the paint breaks. I did all the work on the trailer like that.