Rust at top of tank (inside)

[muzzy]

My CB750 has a very fine /dust rust inside the top of the tank, Its only where the fuel level has not reached.
I have removed the tank and sloshed fuel around I cant see any discoloration or sign of the rust coming off.
The rest of the tank is perfect only area is the very top of the tank that doesn't see fuel.
Any comments or suggestions much appreciated

 

[kirky]

i used white vinegar and salt