Lay down a dropcloth along the perimeter of the fence. Use a wire brush to gently scrape away any loose paint, rust or existing grime, but do not scrape the surface too hard, as it will damage the metal itself. The goal of the wire brush is to remove loose materials, not to remove every bit of paint on the fence.
Use a rust-inhibiting spray on any bare spots that have been completely stripped of paint to protect the aluminum from moisture damage.
Take a paint roller and gently roll it into a paint pan filled with paint. Acrylic paint is recommended to aid in the rust-prevention process. Make sure the paint is evenly applied to all parts of the roller. Using the front of the pain pan, roll off any excess paint so that there is only a thin layer of paint remaining.
Move the paint roller along the aluminum fence. Because the fence likely has many gaps, you will have to move slowly to ensure that all parts of the fence are being hit with the roller. Be sure to reach the roller into deep corners and tight crevices of the fence. Continue until both sides of the fence have been painted.
Allow the paint to dry overnight. The next morning, touch up any areas you may have missed and remove the dropcloth.