Guys, I am a in industrial water treatment consultant with a background in metallurgy for 25 years.
There are two four types of "deposits" that you will find a pontoon like this. One is organic...i.e. biological. The second is scale. The third is the result of an electrical chemical reaction....basically a corrosion by-product. The forth option would be something physically put in the toons by a person...such as a coating or perhaps something that got splashed in it. The only real way to know what is there would be to do an deposit analysis. Anything else is speculation.
Organic growth will scrub off and can generally be removed with a caustic-based cleaner or physical force like a power washer.
Scale (not likely) will come off with an acid. The only way you will get scale is if you are in very hard water.
The third issue (most common) is the result of a very natural reaction called corrosion that aluminum undergoes. When aluminum corrodes it reverts to aluminum oxide and turns dark black. This is a good thing....do not remove it. This is how aluminum protects itself from further corrosion. It is protective layer and if you take it off, you are subjecting yourself to further corrosion. Aluminum is SUPPOSED to be dark. Thats what it does.
As to the fourth option....well, unless you would know what it is, there is not point is trying.
For those that use things like coil clean, you are really just trashing the metal.
