craftgrrl | Making a silver flask usable

I have a silver flask, very similar to this (in terms of dimensions):

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3620389969&category=208

I'd like to give it to my SO; however, I've had the darndest time cleaning it! The enclosed instructions said to fill it with water a few times and gently swish. However, this action produced lots of black and metallic gunk. I think I've probably rinsed the thing scores of times, and I'm still getting little bits out of it.

What would be a good way to clean this thing for future use, if use is possible?

Alternatively, would there be a way to seal the inside with something food-safe?

Thanks for any suggestions!