Toss 'em. I use cleaned out plastic bottle tops, like from pop bottles, milk cartons, etc. Use 'em a couple of times then toss 'em. Totally free!
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I went to the craft store and bought some small glass containers that hold a tea candle. They are about 1 and a quarter inches wide and three quarter inches deep to mix my paint in. Super easy to clean up and do not rust.
Start using tea candles and save the aluminum cups,
there are also some brands that come in plastic cups
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See if you have a company that supplies containers for the food industry. Their sauce portion containers are cheap. i.e I can get these here in Australia (container no lids…) for $4.15 AUD for 100 that is around $2.80 USD basically 3 cents each…
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If you have few of them put the rusty one at the top of the pile you will see that the rust attacks the one on top without touching the others … with mine it works, the rust is rather lazy, always chooses to attack the simplest thing, however I bought a few small Lab Borosilicate Glass Beaker and also 2 PTFE Beaker…for me better than any plastic or metal
no more waste or rust,easy to clean,nice spout 
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I think the only thing you can do with that rusted tin is store some diorama landscaping material like dirt in it.
I forgot to mention plastic shot glasses (or medication cups such as is used in hospitals). Available in dollar plus stores usually in a pack of 40 -50 for a couple of bucks.
Thanks for all the answers and I will look into them. Maybe even today. I want to add this to my answer. I would like to minimize waste. And have stuff I can reuse. But, I’ve now learned from experience that it also has advantages to use disposable things. So this might ask for some investigation. 