Thanks for the reply, I´ll try that.
The Oxalic acid I have used it diluted with water (I have powder) to clean the leather before the dye.
EDIT: I have also tried today, in a new strap, oil it first as I saw in the forums and then dyied them in a larger bunk. It looks much better.
I'll look into the diluted dye with isopropyl. I assume I'll get a lighter color.