Sand the entire birdbath, including the pedestal and the inside of the bowl, with the coarse 60-grit sandpaper. Rub vigorously to remove all the rust. If your birdbath is just tarnished and not rusted, proceed to Step 4.
Sand the complete birdbath again with 220-grit sandpaper, going over all the areas you sanded with the coarser sandpaper. Try to remove any traces of remaining rust.
Sand the birdbath with the fine 600-grit sandpaper, especially being sure to smooth the inside of the copper bowl.
Apply metal polish to the birdbath with a soft cloth. Cover both the bowl and the pedestal.
Seal the birdbath with a clear acrylic sealer spray after the metal polish has dried.
Remove any nicks or dings from the copper bowl that may have occurred during a fall. Place a cloth over the bowl, and gently hammer the indentations through the cloth with a hammer to push them back.
Put construction glue on the bottom of the bowl, the part that sits on the pedestal.
Set the copper birdbath bowl back on the pedestal. Allow three hours for the glue to completely set before refilling the birdbath with water.