Like any other surface, preparation is the key to successfully painting steel shelves. Grease, oil and dirt must be removed for the paint to adhere. The less rust that's on them, the better, but even steel shelves with some rust can be painted. Before painting, consider practicing with spray paint on cardboard or scrap lumber. This will help you avoid runs, streaks and puddles of paint.
Wash the Shelves
Break down the shelves as far as you can. This will make it easier to reach all surface areas on the shelves with paint. Scrub the shelves with a solution of liquid detergent and hot water and a coarse scrubbing pad. Use a stiff wire brush for rust. The more oil, grease, grime and rust you get off in this step, the easier the rest of your preparation will be.
Dealing with Stubborn Rust
There's a good chance the shelves will still have rusty spots after cleaning. Use steel wool, sandpaper or a wire brush on the dry shelves to get as much remaining rust off as possible. For stubborn spots, you can try using undiluted white vinegar. Wipe the vinegar onto the surface with a rag. You can also try creating a thick paste of baking soda and water. Cover the rusted areas with the paste, allow it to set for a few minutes without letting it dry out, then scrub the area with a wire brush or steel wool.
Sanding
Once you've removed all the rust that you can, sand the entire surface of the shelves. The more you rough up the surface, the better the paint will adhere. Sanding will also help remove any residual grease and oil stains. Wear eye protection and a respirator mask while sanding. Brush off the dust left over from sanding with a nylon brush, then rinse and dry the shelves. All the dust must be removed from the shelves.
Primer and Paint
Find a suitable area to paint the shelves. It should be well-ventilated with no objects nearby that might be damaged by mist from the spray paint. Lay down old pieces of cardboard or drop cloths. If weather is mild, this is a great job to do outside in the yard. If you need to do it indoors, make sure the area is well ventilated. Your first coat of paint should be a rust-inhibiting zinc chromate primer. The paint you use for the topcoat should be formulated to work with your primer, so be sure you read the labels on the spray paint before purchasing them.
Spray Painting Tips
When you spray the primer and topcoats, lock your wrist and move your arm in a sweeping motion to spray even, steady strips across the surface. Start your arm in motion before you press the spray nozzle. This will avoid accumulating paint in one spot. Also, once you start your sweeping motion, don't stop in the middle of it. Release the spray nozzle before you stop the sweeping motion.