I use a marine primer -- had been using zinc phosphate primer successfully, but I also see reference here to zinc chromate so I will be checking up a bit before the next primer purchase. Actually, I think wire brushing, priming and painting are the best protections -- but it all depends on use/exposure. If you are having a continuing problem, I would definitely install a sacrificial zinc on the ventilation/cavitation plate as well as priming and painting around it. Salt water is worse than fresh water, and leaving a boat in a marina slip is a hazzard because of the potential for galvanic corrosion (need to keep the lower unit raised out of the water).
Yea Bill, it either sounds like you have an electrical problem (something letting stray currents flow to your motor), or you need a new zinc anode.You need to find the source of the corrosion current, or not painting or coating will help you.
Aluminum Corrosion on lower unit
Linear Mode
