The best way to avoid flash rust after a freshwater rinse is to toss a bottle or two of alcohol in the tank, swish it around, then carefully blow dry the remainder with a hair dryer....I say carefully because you dont want to ignite the alcohol fumes with the hair dryer.
Interesting you left the Evaporust residue in the tank. Does it rinse out easily afterwards?
While rummaging around the garage I found half a jug of methyl hydrate and will swish that around if I decide to rinse the tank with water now.
Part of me says I should bite the bullet and put a liner in it now.