I had similar issues. For me I just used a dremel with small wire brush to clean them up inside and out. Rinsed everything out and then coated with LPS-3. Once I started flying and having fuel sloshing onto them it seems to have taken care of itself more or less. A little surface rust now and again, but nothing that worries me. Wish they were stainless.
The old standard way to treat this is to use Naval Jelly, only on the rusted parts. I'd probably apply with a Q-tip and then remove carefully so it doesn't get on anything else.
-- Paul S
Model III SN910
582 IVO Med
I wonder who the mcbeans supplier for these tanks are? A stainless fitting or even a skim of grease on the steel would go a long way here