Painting Over Rust
It is possible to paint over rust but it is advisable to remove as much rust as possible from the metal surface before priming and painting. As with any painting project, it is desirable to make the surface ready by making it as clean, dry, and smooth as possible, so that the paint will adhere to the surface and look flawless. After the surface is adequately prepared, only then can the primer and the topcoats of paint be applied.
Rust can be removed from metal surfaces by a stiff wire brush or by sandblasting. There are rust removal products on the market that can make rust removal easier, but in the end a wire brush and a lot of hard scrubbing are necessary. These come in the form of spays, solutions, and pastes, and are supposed to dissolve the rust or convert it into substances that are easier to remove. These work to a greater or lesser degree. Be sure to follow the product instructions as closely as possible, especially the safety instructions as such strong chemicals can be caustic and dangerous to work with. Sandblasting is easier if you know how to do it and have the right equipment, but for smaller projects and smaller budgets, rubbing off the rust with a small stiff wire brush is the surest way to remove the rust. Remove as much rust as possible from the metal surface without damaging the surface.
Preparing and Painting the Metal Surface
Clean the surface after brushing with a cleaning solution and let dry completely. Brush on a primer especially for rusty metal surfaces. Allow enough drying time depending on the instructions on the can. Use a spray paint of the desired color that is especially formulated to inhibit rust. An enamel appliance spray paint is good if you are painting an appliance such as a refrigerator or freezer. If using spray paint, work outdoors or with adequate ventilation. Cover the floor area under and around the object to be spray painted with enough newspaper to protect the floor underneath from being spray painted. Mask any areas to be left unpainted with masking tape or painter's tape. Hold the spray can parallel to the surface being painted about eight inches from the surface and spray a small area at a time with even, overlapping strokes. Allow to dry for the time specified on the spray can, and if better coverage of color is needed, apply another coat of paint in the same way. Allow to completely dry before removing the masking tape and the newspapers on the floor.