hull maintenance

[Submariner]

I think the oxidation was always there. Compounds contain oils and fillers that can give the surface a shiny look, and once those wash away the hidden oxidation surface is revealed again.

My guess is you didn't cut enough of the oxidation away, and the compound fillers gave a false sense of remediation. I think you'll have to cut again, or even sand it.

The trick is to buff until you think it's all gone, clean the surface and then let it sit for a day or two, then use shop lights from the side and view at different angles to see if all the oxidation has been removed.