Cleaning the scum of my aluminum boat.

[rolmops]

After a season of having slipped my boat in Sandy Creek I took it home to service and clean it, but it seems that the scum is impossible to remove. I tried starbright gel ,slimy Grimy, Toilet bowl cleaner, scrubbing and pressure washing. All the scrubbing is starting to remove some of the paint but the scum does not come off. I am seriously considering having the hull sandblasted, but before going that way, I would like to know if there is some magical cleaner. Does anybody have a magical trick ,or Is sandblasting the only solution?

[Fishmedic]

Try some Aluminum bright.  I get it at NAPA store.  Mix 50 / 50 . Spray on, set for couple of minutes then wash off. Used it for years on pontoon boats.