Re: how much rust is too much for inside your gas tank?
Course sand or small pea gravel will also work. A couple of cups, shake the tank hard until your arms get tired then pour out the sand/gravel and rinse well wth hot water. Compressed air will help dry the tank after the water but make sure you get all the sand out. If you have access to a sand blaster (bead blast) that also works good but again rinse until you know the sand is all gone.
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