Whether or not your piece is made of metal, you can create the look of old enameled iron that has chipped and rusted. This is great for new metal pieces, statuary, clay pots, or anything you can think of. It only takes some spray paints, rock salt, and a soft paint brush.
Step 1: Supplies
Your decorative piece: small statue, resin wall hanging, etc..
Matte finish Brown spray paint
(optional) oil-based hobby paints in matte rusts, browns, reds, and yellows
Gloss spray paint in desired finish color
Gloss clear spray paint
One small detail paint brush
One soft painbrush (old cosmetic brushes work here too)
Step 2: Rusty Undercoat
Basecoat your piece in the brown paint.
If you wish to have a more detailed or old looking rust effect, you can accent it by mixing together various shades of brown with reds and yellows. Look at some photos of old rusted objects as a reference. Using a fairly dry brush, dab the shades of brown randomly in areas where you want the rust to show.
Step 3: Placing the Rust
Once the rust basecoat is dry, dip a small paint brush in plain water and dab areas you want the rust to show.
Before it has a chance to dry, sprinkle salt in the wet locations, brushing away excess as soon as possible. Alternately, you can lay piles of salt in places where you want the rust to show and then use a brush, toothpick, or your finger to drop small amounts of water onto the salt. The wet salt will bind lightly to itself and mold itself along the surface and help it stay in place a little better. Let the water dry.
Step 4: Adding Enameled Layer
Once the water and salt are dry, carefully spray the surface with your final color. To prevent powering off the salt with the spray, be sure to hold the can fairly far away (12 inches or so), at least at first. Do several light coats, allowing the piece to dry between coats, until you've built up a solid layer of color.
If you find that the final surface needs more sheen, spray it with a coat or more of the clear gloss, Do this now because you won't want the rust effect to be shiny. Let dry.
Step 5: The Reveal
Once all of the paint is dry, begin removing the salt and revealing the rust below. To do this, carefully brush the salt aside using your soft brush.
When it's complete, you should have a shiny enamel-like color with spots of dully rusty brown peaking through.