Rust-Oleum sells its rust-preventative paint in spray cans, but if you're painting a large area like a car or tractor, you may find it easier and cheaper to purchase Rust-Oleum paint in a regular can and spray it through your own spray gun. The thickness (viscosity) of canned paint works well for brushing, but if you plan to spray it, you'll probably need to thin it. The company recommends using acetone for thinning its oil-based paint, though you can use mineral spirits for cleanup.
Pour 1 gallon of Rust-Oleum oil-based paint into a bucket that holds at least 1½ gallons, to allow room for stirring.
Measure 6½ ounces of acetone in a measuring cup, pour it into the paint and mix it thoroughly with a wooden stirring stick.
Test the mixture by filling the spray gun and spraying a small amount on a scrap piece of metal. You may also test it using a viscosimeter, also called a viscometer. Following the viscosimeter's directions, pour the recommended amount of paint in the cup and time how long it takes to drain out. Compare the results to the viscosity recommended for your spray gun.
Add more acetone or more Rust-Oleum to make the mixture thicker or thinner, measuring the amount you add. Write down the amount you add so when the viscosimeter reads correctly or the paint performs the way you want in the spray gun, you can thin future batches by the same amount.