If you have a sheet of steel and you want to add a patina to it, you can wait for years as oxygen interacts with the metal and creates a patina, or you can attempt to create your own patina. There are a range of ideas that can help. Check out these possibilities:
1. Decorative metallic paint
Rather than waiting for a patina to occur naturally, you can create the look of a patina using decorative metallic paint. This paint comes in finishes such as silver and gold, but it can also come in rusty colours. Make sure that it is designed for use on ferrous metals, and then, use spray paint rather than regular paint.
If applied correctly, the spray paint should create a textured look that allows the steel to shine through, rather than the solid paint look you expect from traditional paint.
2. Rust-activator paint
In most cases, steel is an alloy that contains iron. As it contains iron, it is called ferrous. Keep in mind that iron is abbreviated as Fe because that is short for ferrous. To that end, if you have a sheet or steel that has high levels of iron in it, you can use a rust activating paint. Look for one that is designed to work with iron in particular or that says it works with ferrous metals.
Simply paint as usual and then wait while it activates a rusty patina on your steel.
3. Homemade rust activator
In lieu of buying a rust activator, you can make your own. You simply need the right acidic substances. Mix together 475 milliliters of hydrogen peroxide with 60 milliliters of white vinegar and a couple small spoonfuls of table salt. Then, fill a shallow container with this mixture and place your sheet of steel in it. The acids will break down the protective coating around the steel, helping to activate a patina. If you don't have enough liquid, simply double the recipe repeatedly until you have enough to cover your sheet of metal.
4. Homemade dry rust activator
If you don't want to deal with the mess of a wet rust activator, you can make your own relatively dry rust activator. To do this, you need the same amount of hydrogen peroxide and a few tablespoons of salt. Mix this together, and then pour it over a bucket of sawdust. Mix until you have a dry paste. Then, sprinkle a layer of your acidic sawdust in a plastic container, place the sheet of steel on top of it, and cover with another layer of sawdust.
5. Patina from the fabricator
If you haven't purchased your steel yet, you may simply want the steel fabricator to add the patina for you. The fabricator may have some ideas that have not been covered in this list. Alternatively, they may have access to rust inciting chemicals that you may not be able to get on your own.