Sandblasting your car can remove rust stains and help prepare your car for new paint. However, you must choose the sandblasting media carefully; the wrong media may damage the metal panels of your car, causing small dents or other flaws. When sandblasting rust away, a good rule of thumb for media is to start gentle and step up.
When sandblasting rust, the safest way to remove rust without damaging the rest of the material is by starting with the softest sandblasting media available. One advantage to using abrasive sandblasting to remove rust instead of chemical cleaners is the lack of harsh chemicals and pollutants involved in sandblasting.
For gentle rust sandblasting, ground walnut shells are a soft and environmentally friendly sandblasting option. Walnut shell sandblasting media does not blast the metal, but for some heavier rust jobs, walnut may be too soft to come through.
Aluminum oxide is another gentle abrasive; in fact, aluminum oxide is so gentle that it is used on skin for microdermabrasion. Aluminum oxide packs nearly the power of stronger abrasive sandblasting media like glass beads, but is considerably cheaper than other forms of sandblasting media. However, aluminum oxide is not biodegradable and must be disposed of properly after use.
Silicon carbide is another popular anti-rust abrasive used both in sandblasters and in abrasive rust-cleaning creams. Silicon carbide is more expensive than aluminum oxide.
Stronger abrasives for sandblasting include glass beads and steel shots. Stronger abrasives can be used on cars with thick metal panels that are resistant to damage from the stronger abrasive media.
Steel shots are tiny balls of steel used as an abrasive; they can also be called steel grits. These materials are generally not recommended for removing rust because they are overkill—the strength of the material will remove the rust, but it could damage the metal. But if you have been unable to remove rust with a gentler form of sandblasting media, step up to a stronger form of sandblasting media to finish the job.
No matter what you do, make sure your chosen media is dry before sandblasting. Media that are even slightly damp can clog your sandblaster and stop up operations. If you are using a compressor-powered sandblaster, keep your compressor way out of the sandblasting area to keep from getting even the smallest amount of sandblasting media caught in its intake. This is especially important when the media you use is very fine and can stick to your clothes or blow toward the compressor.