Rust is a form of oxidation which occurs when metal is exposed to oxygen and water. The combination of water, air and metal creates an acid that eventually eats away at the metal and produces rust. It is important to keep metals protected with a good solid coat of paint. Painting a rusted shed is time-consuming due to the need to treat the rust before you can actually apply a cover coat.
Sand the rusted areas as smooth as possible before you prime and paint the shed. Use an 80-grit sandpaper to remove the heaviest portion of the rust. Use 120-grit sandpaper to smooth the areas previously sanded with the 80-grit sandpaper. The 120-grit sandpaper will provide a smoother surface to paint.
Wipe the sanded areas with a cloth that has been moistened with paint thinner. Do not use water. Water promotes rust.
Prime the sanded and cleaned areas with a rust inhibiting primer. Latex paints and primers are rated for the particular type of metal that you are painting. Know what type of metal that you are painting and read the label carefully on the products that you intend to purchase. Primer will dry very quickly. Most can be cover coated within 2 hours. You should only need one coat of primer on the rusted areas.
Apply a rust inhibitor to coat the entire shed, if there was extensive rust. Some inhibitors require no sanding or scraping. One type of rust inhibitor will convert the rust into magnetite. Magnetite is not affected by the combination of moisture and oxygen, so you can be sure your shed will not rust again.
Use an airless sprayer, brush or roller to apply the inhibitor to the entire surface of the shed. Allow the shed to dry overnight before painting. There is no need to prime-coat the rust inhibitor.
Apply a good quality exterior latex paint as a cover coat. It is possible that you will need two coats of paint, depending on the colour that you choose. Allow your first coat to dry for a minimum of 4 to 6 hours before applying the second cover coat.
Using the no-sand rust inhibitor will speed up the entire process of painting a severely rusted shed. Some of these products are expensive, but over time they will pay for themselves.