Found this recipe on the web a couple years ago and is supposedly what is in Captain Phab.
Buy Oxalic acid powder from the drugstore, add 3-4oz of the powder to a gallon or so of water, spray on with a pump sprayer and wash off with garden hose or power washer, $20 worth will last about 5 uses, works good on aluminium boats too. . It also works to clean your wooden decks.