A coat of rusted steel (sometimes called a patina) is an excellent addition to any casemod with a weathered theme. Although you can paint on faux rust for a decent imitation, I personally prefer the real thing. Fortunately, applying rust to a steel surface is quite quick, easy, and affordable.
The first step is to expose the bare steel. Most computer cases have powder-coated steel frames; use some 120 grit sandpaper or aircraft paint stripper to get down to the metal. Next, clean off your surface with windex or a similar household cleaner, to remove any oils or contaminants.
To make your rust mix, gather some some hydrogen peroxide, salt (iodized or non-iodized), and vinegar. These ingredients are very cheap, and can be found in an average dollar store. Mix 8 oz hydrogen peroxide, 2 oz white vinegar, and 1 tsp salt. Mix well, and keep covered when not in use. Hydrogen peroxide is very slowly decomposed by light (hence why it’s always sold in a black or brown bottle), so be sure to keep it in an opaque container for long-term storage.
Apply a small sample of the rust solution to the steel. If there are no strong signs of rust after 30 minutes, you will need to remove the steel’s galvanization. Bare steel is frequently galvanized (coated in a thin layer of zinc) to protect against rust. If there is rust, skip ahead one paragraph.
To remove the galvanization, you’ll need to soak the item in an acid, like vinegar. It is possible to mist on the vinegar, or cover the item in vinegar-soaked paper towels, but I find completely submerging the item to be the easiest way to get a consistent application. Buckets, tupperware, and even thick plastic bags all work well. Small bubbles will form on the surface of the steel as the zinc coating is dissolved. Let soak for around 24 hours, then rinse with water and dry.
Once the steel is ready, apply a thin layer of rust solution. Be sure to put down some plastic drop cloths or newspapers, as the resulting rusty liquid can cause some nasty stains. After a few hours, rinse off the metal and check the resulting coat. If rust is patchy, you need to remove more of the galvanization, or there are still oils or paint residues protecting the steel. If the rust is too light, do another round of rust solution. This technique can also be combined with tape stencils, if you want to precisely control where the rust occurs.
Once dry, the rusty steel can stain other surfaces that it comes in contact with (like clothes, for example). If this will be a problem, I recommend applying an off the shelf clear coat, like Rustoleum’s Satin Clear Coat. Be sure to do a small test patch before coating the whole object, to make sure your particular brand of clear coat adheres properly and looks good.
There you go, a cheap and quick way to add a unique look to any computer case!