I couldn't say whether the pin holes are corrossion or something else like a defect, though my first guess would be corrosion. That said, most that I have seen is surface based and tends to grow across the surface and not deep pitting, like with steel. Others with more experience may add it.
The hole is filling with polishing compound that then over time is reacting to something (likely from the atmosphere) or corrosion below it is causing it to ooze out.
If you have it painted, the corrosion can be stopped chemically, so there should be no need to sand down the pits. Just be sure that your painter takes steps to perform this treatment.
Digging deeper into the surface doesn't help. Typically, once the corrosion starts, the initial layer of corrosion protects against further corrosion. I suspect that the polishing compound left in the hole is helping to trap moisture, which accelerates the corrosion and supports corrosion beyond the initial layer. Guys that leave the blue plastic on their aluminum sees this on the corners or edges where the AL touches the ground in damp environments. It would probably help to clean out the compound out of the pits with mineral spirits after polishing.
RV-6A / IO-320, Flying as of 8/2015
RV-10 in progress
Last edited by lr172 : 12-05-2018 at 02:14 PM.