I bought some "corrosion block" based on readers reviews in the PBO forums and have simply removed the larger flakes of rust, and liberally applied a good squirting.
It gets fantastic reviews if you look about.
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But should I have spent more time cleaning up the engine surfaces first? Is it effective if you just spray it directly onto a rusty surface?
I'd definitely have cleaned up a bit more, but as is so often the case (and I'm no exception here) my engine is pretty inaccessible.
Cheers,