While grocery shopping last week I saw some little tart tins, 4 for $2. They were bright and shiny and I had an immediate urge to rust them up! I've only tried rusting a few times, so I am no expert but I'll give you a rundown on the process I used.
Make sure your item is clean and free of grease. Even oil from your fingers can disrupt the process.
Put a healthy dose of vinegar and a good sprinkling of coarse sea salt (regular table salt is ok too) in a shallow glass dish. Place your item in the dish and sort of splash the liquid all over. Leave it for an hour or so, then turn the item over and splosh that salt and vinegar all over it again. Do this a few times, then leave overnight. My tins were tarnished, but not rusted at this point. Note: If you look at the sides of the tart tins below, you will see a shiny band. This should give you some idea of how much liquid I used.
The next day I added hydrogen peroxide to the vinegar/salt mix. This time I brushed the liquid on and let it dry, brushed more on, let it dry... and so on until I achieved the desired effect. The ones pictured below have about 4 coats of the solution and were left to 'age' overnight. Note: You can also use a spray bottle to apply the solution.
A few words about the process. Safety first. You might want to wear gloves, and avoid getting the solution near your eyes, nose or mouth. Over the counter hydrogen peroxide is generally pretty safe BUT even domestic-strength solutions can cause irritation to the eyes, mucous membranes and skin... so be careful! DON'T add other ingredients if you aren't sure it's safe. Heaven forbid you start some kind of chemical reaction in your kitchen! Keep your area clean and make sure it does not come in contact with items you do not want to be rusted.
And there you have it! Instant old ;)