Once all the damage is gone, go back to handsanding from 180 to 1000 or as high as you can find.

Ok, thanks for the encouragement Bruno.
About the hand-sanding, for now I can only sand it up to 600, can't find any higher.
I went to an American Arme Stock to search for chromium oxide but they don't seem to have it here in my region. Any other proposals where I could find it ?

When hand-sanding, do you sand in a circular movement ?