Is it just rusty or does it have varnish/gum in it? Also, how quickly you want it done...if not in a hurry and it's just rust, I will fill it with apple cider vinegar, drain it and strain it once a day for 3 or 4 days(if pretty bad - never worked on a tank with just a bit of pitting). Then I'll do a cycle of pretty dirty evaporust, then another of clean evaporust. It's all reusable so I have two gallons of vinegar, two gallons of older evaporust and one gallon of clean stuff. I only use the clean stuff once it's rust free, it acts like a sealer and will stop it from flash rusting. Then I'll use red coat when I'm good and ready for it.
The vinegar is like $4-5 a gallon,, the evaporust is like $28 for a gallon but you can use a 20% off coupon from harbor freight and the red coat is like $25-30 off eBay for a quart which is good for up to a 5 gallon tank.
If the tank is really gummy, I'd actually put gas or pint thinner in first, then flush it with water - then start the above cycles. I jacked up my two gallons of evaporust tryna clean out of a tank that was super gummy - so now I use it as a in between cycle. Maybe a bit overkill but whatever.