Cuprum-74 said

Pretty much, Evaporust's other main ingredient is phosphoric acid,

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The Evapo-Rust label does not list ingredients, but an E-Bay seller describes it below so I'm still not sure what the secret ingredient is but I know it does not "burn" my bare hands:​

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CRC Evapo-Rust, Heavy-Duty Rust Remover, Reusable, Acid-Free, Non-Corrosive, Water-based, 32 oz, Removes Rust to Bare Metal​