Hi again, sorry for the late answer but, time ... is not my best friend and still my greatest problem.
So I will answer here all other questions because it looks that my "sponge technique" made a lot of curiosity, actually I believe it´s a amazing technique and can be used in a lot of materials, not only leather coats.
Basically I use a sponge instead of a brush, I use it as a stamp to leave irregular marks on the surface I want, ad at the end I finish with a brush for details.
But, of course there must not be so simple, so ... I think the best way to explain is making a SBS here on PLANETFIGURE so stay tuned I will post that as soon as I can
PS: meanwhile you can check other application I made with this technique on a Viking shield in EUROMODELISMO the link is under