Even if you are making a temporary solution it is always best to remove as much as possible with a rust eating solution such as evapo rust or Rustyco
Watch out for rust converters like Fertan - they can do a good job but the penetration might not be as good as you hope so the rust carries on underneath the crust of converted corrosion.
The converters are a little bit like "encapsulators" these too try to form a barrier that is meant to contain the rust and prevent it from spreading.
I've used the Eastwood encapsulator with good results
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