Hi Fernando
If I am ageing wood I use potassium permaganate, this is a very strong oxidiser which mimics the years of exposure of wood to weathering
It is used in fungus removal so can be purchased in chemists in Ireland. It might not be available in Portugal but give it a try, some times farm suppliers can stock it as well.
I you do get the compound let me know and I can give further instruction
Regards and nice to see how you piece would have looked
Ken
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