To paint metal that has rusted, remove the rust with a wire brush, a drill fitted with a wire brush attachment, or a sandblaster. Wash the item with soapy water, and apply a degreasing agent, such as denatured alcohol, to the surface. After the surface is dull, clean and dry, apply rust-inhibiting primer and paint to the surface. If the metal has extensive rust, apply a rust-converting primer to it.
After removing the rust with a wire brush or other abrasive method, sand the surface of the metal with 80-grit sandpaper, and sand the surface a second time with 120-grit sandpaper to create a smooth finish. Before applying primer, fill holes and other imperfections with a recommended filler if a highly-polished surface is desired.
Apply several thin coats of primer to the surface, rather than one or two thick coats of primer, and allow the primer to dry between coats. Paint the surface with rust-inhibiting spray paint, water-based paint or enamel paint after the primer has dried completely.
When using a rust-converting primer, it is not necessary to remove the rust. Instead, brush the loose rust and debris off the surface before applying the primer. Rust-converting primer is recommended for projects in which a highly polished or fine finish is not necessary, as the primer is very thick.