The Rust-Oleum Semi Gloss Enamel is an Interior/Exterior Paint, Oil, Resin Type Alkyd Enamel. It is fast drying and can be used in either Indoor or Outdoor applications.
You should be fine using it as an outdoor application.
This is from one of the sites and the way it is advertised:
Rust-Oleum® Protective EnamelsRust-Oleum finishes are durable, tough, excellent hiding air dry finishes for interior or exterior protection of most metal, wood, or masonry surfaces. Rust-Oleum coatings resist weathering, salt air, moisture, mild chemicals, and industrial fumes. Prime metal surfaces with model No. 7769 Rusty Metal Primer or model No. 7780 White Clean Metal Primer. Rust-Oleum finishes cover 350 to 450 sq. ft. per gallon, dries to touch in 2 to 6 hours and to recoat after 24 hours. They may be applied with brush, roller, or spray. Thin sparingly for spray. If thinning is required, use paint thinner or mineral spirits. Clean up with paint thinner or mineral spirits. Gloss sheen except where specified. Available in flat, semi-gloss or satin.
This is the same for the Spray product.