Rust is rust. Unless removed, it will come back. I had it happen with my SR500. Besides it would likely cost more to buy someone else's tank, that might have rust started, than to have a truck radiator/fuel tank shop wash out and seal one's own tank.

I will also tell you it isn't much trouble - at least in my case. I drove about 8 miles to deliver the tank, then went back to get it. Not much trouble there. Only way it would be easier would be if they picked up the tank or maybe came out, took it off, did the work and reinstalled it. We used the company to coat some non-current bikes we got back in the 80s that had rust from sitting so long in the crates in the warehouse. Several 1981 CB750F models specifically. That's where I got the idea.

By the way the Red Kote is a hard shell material impervious to gas and rust through. POR15 uses similar if not the same stuff. Kreem is junk compared to it. I did Kreem in my SR500 tank. It lifted and messed up. Had the tank stripped and coated by the radiator repair shop - $35. I'll never do coating myself again. I'll leave it to the pros who can clean and coat the tank properly.