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?

You should rinse it with light oil or diesel fuel. Then, get it back on and get some gas in it.

Well when you remove all the protection from the metal like you did then what would you expect it to do?

Acid etches metal, or down right eats it. If you were stupid enough to put acid in your tank, you should have been smart enough to neutralize it with baking soda or similar basic material.