Time to give the damaged area an acid wash. It's a bit complicated but will work. You must do this out of sunlight in the early morning or late afternoon.
Grab some cheap
dishwashing gloves, a cheap paint
brush, a Scotch Brite pad, a bottle of muriatic acid and a plastic tub. Get a hose on deck
and ready to use fast. Put on the gloves then mix 1 acid : 3 water
. Brush on to the damaged area and wait 2 minutes. Hose off. You may need to repeat on the worst areas and use the Scotch Brite pad. Hose off with plenty of water.
Throw away the brush, pad and gloves.