If the
patina came off with a regular toothbrush then it was very likely of the "doomed
type".
If you come across something similar you can try to leave it
as is. Sometimes a gentle rub between the fingers helps. It sometimes helps consolidate the soft
patina - maybe from the
bit of natural oil on your fingers.
In any event don't beat yourself up over it.
If you even get access to Renaissance Wax (
Ren Wax) or other microcrystalline wax it might improve the overall appearance of this one.
Shawn