Keep it off the exhaust and catalytic converter, brake rotors, pads and calipers, oxygen sensors, and parking-brake and suspension moving parts. Mask those areas before spraying.

Pressure-wash and degrease the frame, let it dry completely, then remove loose rust and scale with a wire wheel or scraper. Treat any remaining rust before applying a coating.

A rust converter chemically turns rust into a stable, paintable layer—useful where you can’t get down to bare metal. A rust remover dissolves rust back to bare metal, which then needs primer or a protective coating quickly to prevent flash rust.

Oil-based coatings creep into seams and don’t crack, but wear off and need regular reapplication. Rubberized coatings are thicker and resist chips, but can crack and trap moisture if applied over rust or poorly prepared metal.

Oil- and wax-based undercoatings are typically reapplied every year, ideally before winter. Rubberized and hard coatings last longer, but should be inspected yearly and touched up wherever they’re chipped or cracked.