You don't need to clean the cylinder... but if you ever plan to use it for breathing gas then you should purge all of the argon out of it. That is, empty the argon and then fill with 50psi and empty that, repeat several times. I was going to say, "shake well between purges" but you probably wouldn't have know I was only kidding.
In any case... argon bottles must be labelled as such and if you convert the bottle to *breathing gas* then the label must be removed. You don't want to create a situation where someone might end up breathing argon!
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Addtional note regarding argon useage:
Speaking of purges... argon suit inflation gas won't help at all if your suit hasn't been pre-purged with argon to begin with. If you are using argon you should fill your suit with it before the dive and then open your neck seal while squatting to get the gas out of the suit. Repeat several times.
It doesn't make any sense to add a little bit of argon to a suit full of air... no benefit will be realized. However adding a little bit of air to a suit full of argon will not unduly eliminate the thermal advantage of argon.
With a suit full of argon you
will stay warmer during the dive though you won't necessarily
feel warmer unless you have the placebo effect working for you.

Even a thermometer measuring core temp won't notice the difference (right away at least) since we are warm blooded and our bodies compensate by burning more fuel to keep our tempurature constant. However the argon
will keep you from losing as much heat and your body won't have to work as hard to replace it.
Make sense?