HOLY sh**!!!!!!

That is F'N NASTY!!!!
If that was in the tank, i can just imagine how corroded the carb is.

(i see some "Sea-Foam" coming into your life.

)
(THAT carb may need some "chem-dip".

)
I would just clean the tank real well, 2-3x FOR SURE.. I have found that pouring a touch of gas in the tank and left to sit, the gas will break some of the rust/debris out, then wash again. I don't have that spotless tank ppl want but it works just fine.
It seems the link you found may be some dam good stuff. Sounds great anyway.

Maybe Z will chime back in on that since he is a boat/marina man. (i had never even heard of it till now.... I always had to use "Naval Jelly" to remove rust.

)