Regards weak tell tale. Did you remove thermostat and clean the housing too? You'll need a new gasket if you take it off. Its normal to get a build up of salt here which crystallises at the end of the season regardless of flushing. Run it to operating temperature with muffs, etc.
You could also check the cooling water jacket at the side of the engine, again you'll need a gasket if you remove to inspect. Use a scalpel or razor blade to remove old gasket material, and torque bolts in sequence - however I'd only do this if you get an overheat alarm.
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