Evapo-Rust with Sodium Nitrite rinse
Evapo-Rust is a great rust removal agent. Submerge a rusty part in it for a few hours and usually all the rust is gone without any staining or etching of the part.
However, after removing the part from the solution, some gummy deposits tend to form which are hard to remove. I tried wiping with a clean cloth, alcohol, acetone - without much success.
After drying even water has a hard time cleaning the deposits. But that is basically no different from rinsing the part with clean water after derusting. Naturally, water on a clean piece of steel tends to cause new rust, especially in holes.
In the past, I washed off the part with water and then used a water displacing agent such as WD40. This works, but sometimes you don't want WD40 on the part so that has to then be cleaned off with something else.
A solution is to use Sodium Nitrite, NaNO2. This is a food additive that has been used as a rust proofing agent for many years.
Published concentrations vary greatly:
I did prepare a 5 g/L solution, cleaned the plate pictured above in another Evapo-Rust bath and then used the nitrite solution to rinse it off. The results are excellent. No signs of rust or deposits at all after the part has been air dried.