Today while smelting some wheel weights I clearly had a zinc one sneak into the batch. I had to trash the whole thing and in disgust I trashed the rest of the wheel weights as well. Scrubbed out the pot and then sanded it clean. It's stainless pot from goodwill and not my expensive casting pot, but I think I got it all out.
It's just that I have reached the point of diminishing returns with wheel weights. You have to sort a big batch to come up with a small percentage of lead and then this happens. I don't think it's worth the time it takes any longer.