Wipe this over with methylated spirits to degrease the surface. Repeat if necessary. Priming of the leather is not required unless you want a white base coat of Acrylic Gesso Primer. A white base will give a brighter design. A coloured base can be achieved using Clear Gesso Base.
Finity metallics are also popular.
If you want to thin the colour don't forget to use medium as well as water.