There are many different point of view on the topic of tank cleaning and sealing.
I like to use phosphoric acid to derust (the same thing that Por-15 specifies.) It does a good job, is fast, doesn't need to be neutralized other than rinsing, and is pretty safe. As a bonus it's cheap, readily available, and it leaves the metal with a protective film that helps ward off (but not eliminate) flash rust when rinsing.
There are other good derusters like evaporust and the like. These are actually top of the heap in my opinion, but they are more expensive and slower.
Vinegar also works but it much slower. It's cheap and easy to dispose of so those are two good bonuses.
Using something strong like HCl works but is more risky. It needs to be neutralized and disposed of carefully. You can expect rapid flash rusting after rinsing too.
If the metal is solid and strong, with minimal thinning, and you are able to get the tank 100% free of rust I'd stop right there and call it done. If the metal is thinned and or springs a leak then I'd go the liner route.
Por-15 is thin like a thick paint and is easy to work with. This would be my first choice unless there is significant metal erosion. In that instance I'd go for Caswell's tank sealer. It's super thick and flows like cold honey. I recommend thinning it to the max allowable on the label. Because it's so thick though it helps reinforce thin metal, include sealing leaks. Tape over the leak area on the outside of the tank before sealing.
When sealing it's critical to flow the material all over the tank to get every inch of the surface covered. Then I'd go over it a second and third time. Then dump out the excess. There will still be extra material in the tank so keep turning it until the material kicks off and stops flowing. Only then can you rest and put the tank down.
It's a lot of work but done right a sealed tank will last for years.