Fly rust occurs when iron particles (usually through brake dust from the car's brakes) get stuck in the car's paint and create surface rust. Regular washing with shampoo and sponge is not enough, so this is where an aircraft rust remover comes into play. A fly rust remover effectively dissolves fly rust - Spray, wait for purple, and rinse. For stubborn ingrained/burned-in dirt, you should instead use acid-based products. Use a brush and rough up the dirt for best results. The products are manufactured in Sweden.