Probably surface degradation. If it's not too bad take a wire
wheel in a driil
motor and clean it to bright casting. wash with alcohol and use the two part epoxy called JB weld liquid
steel. You may have to do a little bit at a time so it won't sag. Just wire brush between build ups. sand smooth, cover with epoxy primer and then
paint.
I was a skeptic over the years of this product but I tried it on a
Westerbeke thermostat housing as a temporary fix with a
corrosion hole until I found a decent priced replacement. I found one but procrastinated on replacing it and when I
sold the
boat 6 years later, it was still doing it's job.