Take the bed frame apart and set it on a drop cloth or covered surface. Have wooden sawhorses or other structures nearby to prop up the frame pieces.
Roughen up the metal surface using 60-grit sandpaper. A sanding sponge also works. If the aluminum was polished, it now looks dull and the shine should be gone.
Wash the surface with trisodium phosphate on a sponge or cloth. This clears off any sanding dust and removes residue.
Apply a metal epoxy primer to the aluminum frame. This is available in spray cans, which is easier to apply on this type of surface than brushing it on. Spray it on, holding the can 12 to 18 inches from the surface, using a smooth, even motion. Allow the primer to cure according to the manufacturer's instructions.
Paint the aluminum with 100-percent acrylic paint. Use a small paintbrush to cover the bed frame; the bristles allow you to work the paint into detailed areas. Allow the paint to cure according to the package instructions. Apply a second coat.