best way i've found is to just blow HP air straight from a tank into the drying tank. I have a fill whip with a junk hose and shove it up to the bottom of the tank while it is inverted. From there I open the valve and slowly pull the hose out. Never had a problem with steel or aluminum tanks
I do the same thing, except I also use the Rust Inhibitor.
Just don't be a dumbass like me. It once took me three times of scratching my head wondering why I was getting such horrible flash rust until I realized I was blowing it out with doubles filled with 36% not air.