be a little more specific as to where the coolant leak was coming from and to be honest the only way to know for sure is to run it (after you remove debris via shop vac or removing the head). and sounds to me like it was just coolant seeing as to it had RUST. rust generally wont happen unless moisture is present in the environment (coolant leak) and after a few good combustions on that cyl the rust would be gone does that help any ?