Why did my gas tank start rusting after I washed it out with muriatic acid?

I have a 1978 Honda Express 49cc moped, the gas tank had rust collecting in it and I ve tried everything before going to muriatic acid. When I drained the tank out I was surprised, it was super clean, and I was happy. Until I came back out four hours later to see it had started rusting again. I rinsed it with water, blew it out and sloshed it around with gas. Why does it keep rusting? Do I need to use a sealant? What should I do? Please help me!

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