How to properly clean a whiskey flask?

I'm answering my own question because I did a bunch of stuff from a lot of resources, and collected valuable information that more people may use in the future when coming across this post.

I wanted to use household items. My flask is a stainless steel with some copper.

What I actually did:

1. Cleaned with a small amount of tap water, shaking it a little bit;

2. Boiled a glass of water, and a bottle of vinegar/water (50%/50%);

3. Cleaned the flask with the boiled water, them with the vinegar solution (50% of the flask, shaking it a little bit). Then cleaned again with the boiled water (50% of the flask), shaking it, them with the vinegar solution again, then with the boiled water (50%), then with boiled water again (this time 100%). I didn't emptied the flask from this last cleaning;

4. I let the water inside for 24h;

5. 24h later, I opened the flask, and emptied it in a glass cup: there wasn't any solid particles, and the water didn't have any taste, so I believed it was ok to put some whiskey.

Maybe your flask is an old one, with leather, I don't know. In this case, I suggest you check these resources:

http://www.whiskywhiskywhisky.com/forum/viewtopic.php?f=16&t=4254

http://www.thetfp.com/tfp/tilted-knowledge-how/29045-cleaning-inside-antique-hip-flask.html

http://ask.metafilter.com/111820/Mildewsmelling-hip-flask-can-it-be-salvaged

http://forums.overclockers.co.uk/showthread.php?t=18080329

If there's no evil smell emanating from the flask, just give it a good rinse with hot water. Otherwise, a 20 minutes soak in a dilute solution of bleach (around 1 tbs per gallon), followed by extensive rinsing with warm water should do the trick.

Is the flask actually stainless, or merely something that's been plated bright and shiny? I'd go w vigorously shaking some sand around inside of it, to loosen any corrosion/crud deposits. Follow that w hot water/detergent, and brushing as possible. Finally, give it a soak in Vodka/Everclear. Check to see that the rinse solvent comes out clear and without chunks. For all you know, someone from the 1990's left Cream liqueur in there to evaporate.

Steradent tablets are best for cleaning any type of flask in my experience.

There are a number of flask and bottle brushes available online that will help with this task. I also found this set of instructions at ehow that recommends the use of boiling water and distilled white vinegar. Unlike most cleaning products, white vinegar will not leave a lingering scent in the flask.

I have always had a lot of success using uncooked rice as an abrasive and a dish soap/hot water combo. Shake it like crazy and it's clean. It's a safe, simple, and effective method.

Great question and some really good answers.

I am the founder/craftsman of a company that sells flasks and we recently wrote a blog post on it that can be found here: http://1776.co/blogs/journal/54690755-how-to-clean-your-leather-whiskey-flask

The short of it is:

Use warm soapy water and allow it to air-dry. Your flask should always be washed by hand because the chemicals in dish washing detergent may react with the stainless steel interior.