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
6 years later, it was still doing it's job.