Reuse of Food Grade Tins

The problem is that many 'food grade' tins are coated, and not solid. As such, cleaning them can end up flaking off the coating that's been applied, making them reactive.

Unless you're dealing with antique materials, I suspect that the majority of 'tins' are going to be either coated steel, or stainless steel.

Although I mentioned looking for tooling marks, and if it's matte or shiny, an easier test might be to use a magnet -- if the magnet sticks, they're most likely steel, and thus prone to rusting. Even if they're coated, if you put something moist in there, it can seep into any cracks it finds, leach at the metal, and then ruin your food. I had this happen with some Pecan Whiskey Cakes that I had put up, as they need to soak for a couple of months ... I now understand why my aunt wraps hers in (alcohol soaked) cloth, then a plastic bag, then whatever she's going to store it in.

You could probably do the same thing as you're only dealing with a few days -- wrap it in plastic, then put them in the tin.

It's possible that some (non-stainless) steel containers might have a more durable coating ... but unless it's specifically sold as being re-usable (or as wombar mentioned, for the industrial/institutional market), it's likely made to keep the costs down and not intended for re-use.

You should also avoid acidic or salty items, as they're known to pit stainless steels, especially when there's moisture and/or poor ventilation.