Hey Ironpuppet,
You can use plain borax (20 Mule team works fine) as a flux but honestly you don't need to do anything other than skim the cruce as you're already planning on doing.
What are you casting? Pre-heating the molds will help particularly if you have some narrow areas in the mold and/or finer detail - if the piece is larger and solid it's not as important.
A steel tube with a welded bottom is fine - don't overheat the aluminum - it doesn't take long to get to temp.
Glenn's right - if you haven't done it in a while, go overboard on the safety prep.
Let us know who it goes!
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