Found a small rust spot on the roof, how to deal with it?

[Dr Who]

That citric acid works ok and all but when I see some of the stuff people have used detox c on it works a little better imo.
Having said that based on what you have said I will be getting citric acid as its like you say way cheaper

I used both, and they performed the same, it is all down to the concentration, they both even leave the same yellow film on the metal after treatment.

Citric acid is second only to electrolysis imo