Join Date: Feb 2010
Location: Narragansett Bay
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Re: Yanmar 3GMD: Rydlyme or other marine descaler in older engine
I would guess you're looking at salt water scale, not zinc oxide.
Rydlyme is great stuff. You need to fashion a way to fully recirculate the product for a period of time. That may require pulling the thermostat and/or impeller. It depends on where you plan to enter/exit.
It was so effective on mine several years back that the scale clogged the pump. A filter of some kind would be a good idea, even if just a strainer than return fluid dumped through.
The exterior corrosion appears to be from leakage, so I'd try to seal those back up again. I can't tell from the pics, but if the paint had been compromised in those locations, they may simply be exterior corrosion.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.