Prepare the item by applying one to two coats of base primer to coat the entire surface area. Sprinkle fine grain sand over the item, making sure to cover areas where you want the fake rust to appear to have the heaviest concentration of sand. Allow the primer to dry between coats, then allow the entire piece to dry overnight.
Dip a paintbrush in dark brown paint and dab the paint on the item. Cover just over three-fourths of the item, leaving some of the sand-coated primer showing through. While applying the paint, swirl and tap the paintbrush to produce an asymmetrical application. Go back over the item with the red-orange paint. Repeat the process with dark brown, this time making sure all of the original sand covered primer is hidden under the dark drown, red-orange or mixture of the two colors.
Mist the entire item with a spray bottle containing plain tap water. Mist thoroughly and completely. Dip one paintbrush in dark blue paint and another in gray paint. Tap and swirl both colors onto the piece at the same time, directly on top of the watered down brown colored base. The blue and gray will mix in to the red-orange and brown layers underneath, slowly building a rust colored base complete with natural looking striations of color.
Mist the item once more and use a fresh paintbrush to lightly swirl the various colors together in a random pattern. Use a piece of cloth to dab and swirl colors around. Trickle on extra orange or red-orange paint if you want to create sections with deeper shaded rust or corrosion.