IT MAY NOT BE STAINLESS ANYMORE
Sorry for the huge emphasis and the absurd caps, but I'm reading some... odd statements.
I should point out I'm in chemical engineering, and did cooperate with experts in metallurgy.
Stainless steel chemistry
Stainless steel is such because it does not rust, as everyone knows.
Rust, as many know, is the oxidation of iron.
What few know is why stainless steel does not rust: the answer is, chromium.
Steel, to be classified as stainless, requires at least 10.5% of chromium content in mass (not volume, you'll see why in a little).
Chrome oxides on the surface do not oxide further (due to their extremely high oxidation potential), and protect the underlying layers of iron and carbon. This is called passivation.
What only people in the business know is that to produce those oxides, strong alkalis are used (commonly, other procedures are possible).
You've removed the surface of chrome oxides.
This layer is so thin it is invisible to the naked eye, and only crystallographic measures can reveal its status when it is present.
What's easier to see is its absence: rust forms in those points.
Fine grain polishing won't fix your problem.
Looking the same as stainless steel doesn't make it stainless steel, as looking the same as gold doesn't make it gold
If you try to weld some stainless steel, you'll see this process very distinctly: as you lower the chromium ratio adding carbon (from combustion, if you're using it) and migrating iron (by the very definition of welding: you're adding material, even when joining two pieces), rust attacks the welds first, to later spread under the passivation layer to the whole piece. That's the reason MIG and TIG welding techniques were invented, and Friction Stir Welding is making leaps forwards: it works with all metals (as it relies on the metal phase uniformity), requires no chemical and no added material, and does not alter chemical composition (but it does alter the crystal structure).
You said may not be stainless !
It depends on how much material they have sanded: as the chromium lies mostly on the surface (hence the reason the volume percentage is not important), there may be a veeeeery thin layer left. I wouldn't bet a coffee on it, 'tho. Furthermore, the layer may not be everywhere as thin as required, even being absent in some points.
You can fix it !
As passivation is a process, you can apply that process to your item! Look up a business to do it for you, it shouldn't have a huge cost. If the reinforcing bands at the base are made of wood, those will have to be taken off and then placed back on: ask if they can do that for you, and if so at which price.
Traditionally, a nitric acid bath and further oxidizers were used to
- Strip away rust that hasn't been sanded
- Promote oxidation of the chromium atoms in the alloy, restoring the passivation layer
Too bad that this process was polluting and hazardous. It has interesting interactions (explosions) with some organic compounds, too.
Fortunately today a cleaner, safer, cheaper, more effective solution exists: citric acid.
Yes, it's that citric acid: the citrus juice one.
In high enough concentration, not only is it a food-safe preservative, but also a passivating agent for stainless steel.
The chrome layer, when using this acid, is actually restored by the spontaneous oxidation of chrome in the now-exposed alloy.
Don't even consider a nitric passivation for your brewing equipment