Keum Boo on copper is a bit tricky because copper is easily oxidized when it's heated.
So you need to de-oxidize the surface using a torch.
Flame is an excellent reductant.
The preparation is exactly same as the usual Keum Boo but you don't need to do depletion gilding.
The point is brushy falme and de-oxidization by it.
I mean flame has 3 functions in this case.
1. Heating
2. Eliminating oxygen from the oxidized copper.
3. Protecting copper from oxidization.
When the oxidized copper is de-oxidized, the surface becomes fresh copper-pink. So you can know it's time to push the foil.
In this video I only show the first Keum Boo but actually I did it 4 times including fixing.
After Keum Boo is done, I applied LOS patina slightly and wiped it out a bit and sand blasted and then applied Renaissance Wax.