How clean is enough for fuel tank?

[malbert73]

Hi everyone- my aluminum fuel tank is original to boat and has fore-aft baffle in middle of tank with no access. I was able to clean 1/2 from pickup port but finally cut an inspection hole so I could clean the other half. Emptied fuel out first.
What I found was a thin film of black goo on bottom and 1/2 up sides of tank. Used diesel-soaked rags to scrub and it now looks pretty clean except for some black stained areas and some along seam. See pictures. Should I try to get it all shiny? Some solvent I should use? A brush? I also don't want to scratch surface. The one area has some putting which is very superficial but has me puzzles since nowhere near low point of tank so hence would never see water, and is under aft berth wel above any bilge water or moisture on underside.




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