Thank you for your useful comments, Christoph,
I got some acrylics to go to coloured undercoating, with some grisaille to build the lights and shadows before attacking the oil painting and finishing.
I will try this black undercoating with a next 30mm flat to make use of this lining effect, which I find fantastic for small size to bring a distinct reading of the details ... And perhaps do the same figure with a traditional white undercoating to compare, then a coloured grisaille ...
An interesting experiment ! but without mastering the techniques yet, so as preliminary attempt!
It is useful to try various techniques, to change, discover, improve and share the experience with others !!
Main purpose of the forum, I am convinced.