Stiff Scraper, sandpaper, grinder
By metal surfaces, maybe do you mean the strut, propellor and shaft or maybe you mean the keel. Or maybe the lower unit of your outboard motor?I use a strong scraper with a bent and sharpened edge to get the majority of the barnacle off my keel, strut, and prop. Then I use sandpaper, scotchbrite until the shaft and prop shines. I always install new shaft zincs also but clean the stainless steel shaft to a polished finish before installing new zincs.