I use muriatic acid to remove rust from cast iron pans all the time.Pint to 5 gallons. Pour the acid into the water, not water into the acid... works good. Let it sit for a while . When it smokes you know it's working. Then use trisodium phosphate or some other cleaner and flush accordingly.
Beware..
Muriatic acid eats aluminum for dessert!

~~ Jethro
1954 3100
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1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020