Did you read the instructions? Ferrous phosphate. https://www.rustins.ltd/images/files/1464-PDS-Rust%20Remover%20PDS.pdf
Pffft. Read instructions? Just remove the black stuff with an angle grinder, what could go wrong?
@Bobbo started
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Did you read the instructions? Ferrous phosphate.
https://www.rustins.ltd/images/files/1464-PDS-Rust%20Remover%20PDS.pdf