Hi Desib, i have never had the problem of tannins leaching out of the leather. However, to make and mold a mask, the leather is first soaked in warm water for about 30 seconds and the water ends up a brown color. After molding and drying i then airbrush without any sealers. I used to water down the acrylics, but i found the color was a bit washed out. I have just started to dilute with spirits (metho)and find this works well. However when you mix with spirits there is some sludge at the bottom so i only use the liquid that sits at the top otherwise the airbrush blocks up. How much to dilute will be a bit of trial and error depending on what airbrush you have.
I use Jo Sonja's acrylics....great colors and a very good price...the cheapest price i have found is from Spotlight. Lumiere is also very good, but very expensive but they have some beautiful pearlescents and metalics.
Cheers