Amazon has the 24oz Rust-Oleum Stops Rust Turbo Spray (Rusty Metal Primer, Flat Red) on sale for $13 with free shipping on $35+ or with Prime (extra 5% off with 5+ S&S orders). If buying in bulk, the 6-pack is around $12 per can. 

• Designed for large jobs with 10" tall fan spread for professional spray equipment results and 4X faster project completion.
• Dries to the touch in 2 hours, recoat within 1 hour or after 48 hours - covers up to 20 sq. ft. per can.