Raise the pool water's pH to lower the acid level and to ensure effectiveness of the rust removal product. Add a sodium carbonate-based pH increaser to the pool as instructed in the manufacturer's instructions.
Drain the pool, if necessary, to put the lights and rust-affected areas above the water line.
Turn off the main circuit switch for the pool lights then unscrew, pry up and remove the light fixtures. Stretch the cords behind the lights and place the fixtures on the pool deck. Remove the light lenses and lightbulbs.
Coat all rust stains on the light fixture and on the pool walls with biodegradable rust remover. Scrub the rust stains using a stiff-bristle, mini scrub brush, then leave the rust remover to sit for the time indicated by the product's instructions. Thoroughly wipe away the dissolved rust and residual rust remover with a damp towel.
Equip a rotary tool with a grinding stone bit and grind away any rust pitting left on the metal light frames. Once removed, switch to a buffing bit and polish the frames with polishing compound.
Paint the metal frames with a clear, pool-safe, waterproof metal coating using a small round paintbrush. Underwater metal clearcoat will provide a shield against future corrosion. Allow the coating to dry.
Reassemble the lights and reinsert the fixtures into the pool wall.
Have your pool water tested for substances that may lead to rust. Water with high levels of iron, for example, encourages rust growth. Consult your local pool service person for possible solutions.