Brown said:

A small amount of iron can turn into a large amount of rust flake. As everybody else has been saying, scrape and tap your way to something solid underneath. With a bit of luck it won't have weakened the casting too much.

Many years ago a friend with a boat noticed that the oil in his bilges seemed to be coming from holes that had rusted in his engine. We cleaned them up and stuck patches over them with Araldite. It lasted for ages. Of course, I'd never recommend doing any engineering as shoddy as that

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