Thank you everyone, as always you are very kind.
John, I use a very pointy tip tool, like a pin, and hit the surface, then I hit it with a ball point tip ( round tip, real tiny as well) ... even a ballpoint pen ( bic) without ink will do ... so it is a combination of concave and convex tiny little forms on the surface... that is it... no secret...
the only thing though is you must hit said texture with a bit of lighter fluid and a very soft brush as to round things off, being careful not to destroy everything you've done... also, try to download high resolution reference of face close ups, as to match the rhythm of the pores of the face... that is it... I must warn you it will take hours, so you must be very patient