Spread drop cloths over the basement floor. They will catch paint that accidentally drips.
Set stools or ladders in the basement to help you reach all parts of the duct work.
Combine one part trisodium phosphate (TSP) with three parts water in a bucket.
Clean dust and dirt from the duct work thoroughly using the TSP-water mixture, rags and sponges. Rinse the cleaned duct work with sponges soaked in plain water. Allow the duct work to air dry.
Apply a coat of metal paint primer to the duct work with a paintbrush. Let the primer dry completely, which takes at least three hours.
Paint the duct work a color or colors of your choosing with paintbrushes.