Re: Using Non-Acid Based Coil Cleaner for Cleaning the bottom of fiberglass boat
Some real world experience--
50 series aluminum, which is what they make pontoon logs out of, is completely and utterly unaffected by muriatic acid, at least in my experience. Cast aluminum and 60 series aluminum is another story. I soaked some old props in an acid mix, and got sidetracked, and after about 15 minutes this is what I ended up with:
Do not get muriatic acid on your shiny aluiminum trim which is probably 60 series, and DEFINITELY keep it off your stainless. Muriatic acid will permanently darken stainless (so much for "stainless" steel). I've got stains on my SS rub rail from not being careful. (Also I've stained 2 SS kitchen sinks by using too stong a mixture). Muriatic acid is perfectly okay to use on your outdrive and gelcoat, it's safe on paint, BUT, if there's any rust on your trailer, get it rinsed off quickly. Also, fwiw, muriatic acid does nothing to your lawn.
As for coil cleaner, never heard of it but I already know what's in it: Lye (sodium hydroxide). Oven cleaner, drain cleaner, Castrol Superclean, Purple Power, all use lye as the heavy-duty cleaning ingredient. Lye will immediately emulsify oil, including the natural oils on your skin. Cleans your hands real good, but you'd best be for getting some lotion on afterwards.
Aluminum toon logs-- I've used 3:1 mixed "The Works" probably a dozen times on our Party Cruiser logs, and it does absolutely nothing except burn off any calcium deposits. I use a garden sprayer. The calcium boils and smokes when hit with the acid. Spray, wait 5 seconds (that's the half-life of the acid when in contact with calcium), spray again. When you get no boiling smoke, move on. Works exactly the same on gelcoat. I've found that all I have to do is pressure wash the green goo off, then simply spray the calcium with the garden sprayer, no brushing needed. Just spray and let the acid do the work. Rinse the boat and ground with a water hose often. That's it.
Now, put some purple cleaner or Easy-Off on the logs and wait about 30 seconds, and that 50 series aluminum will turn almost white. The lye etches the aluminum and removes the natural dark oxide stains. I did that once to my logs, and it's pretty scary. That was about 5 years ago and the logs now look like they did before...
For fun, put your coil cleaner on white fiberglass, and prepare to lose your lunch as you watch your glass turn yellow. This is assuming there's enough lye in it and no wax on the gelcoat. I know there'e plenty enough lye in Castrol Superclean to yellow 'glass. However, the yellow is temporary, the glass does return to white. The stuff will probably clean off the algae, but I don't think it will affect the calcium, it didn't on my toon logs...
Last word on lye, I did find, on our old Sun Runner, that straight Super Clean is great at removing the dead oxidation from badly oxidized gelcoat. It doesn't shine the gelcoat, it does just the opposite. What it DOES do is removes all the loose, powdery oxidation particles that just soap and water won't touch, and leaves you a with a better surface to start with if you're about to attempt to shine it up.
That got myself and others at the marina wondering if it would work on our fiberglass bottoms. 
