Rusty galvanised metal can be a problem to paint. In the first place, galvanised metal is chemically resistant to rust. If you have rust, you might have problems that paint can't fix. In the second, the same chemical factors that make galvanised metal rust-resistant make it resistant to adhesion from many kinds of paint. Once you get past those two factors, the painting process is similar to painting any other surface: You just need some specialised tools.
Remove all rust using the wire brush for large patches and the steel wool for detail work. If more than 25 per cent of the structure is rusted, or you find the rust goes all the way through, replace the galvanised metal structure rather than repainting.
Set down dust sheets beneath the area you intend to paint.
Wash the surface you intend to paint using a scrub brush or power washer. Wait for it to dry completely before continuing.
Run a line of masking tape at any point where an area you want to paint meets an area you don't want to paint.
Apply a coat of primer to your galvanised metal, running your paintbrush in smooth, even strokes. Use primer rated to adhere to galvanised metal. Other primer will run and wash off.
Wait for the primer coat to dry completely.
Apply a coat of paint to the primer coat. The nature of primer means this can be regular exterior paint. Use the smooth, even strokes you did with the primer. Allow to dry, then add a second coat if needed.
In most cases, the paint alone will act as a sufficient weather seal to protect the surface from further rust. However, if you live in an area with particularly humid weather or wild temperature extremes, a sealant might be necessary. Check with your local paint store: The staff there will know the best practices for your area.