...technique I read in 'Art of Weathering' by Martyn Welch that I would like to try ...
I've tried the Martyn Welch Maskol method and it's certainly effective. However, while it's great for 7mm, the edges close up can look a tad deep and too distinct for 4mm. So, the jury's out for me, and I've reverted to stippling rust/black/leather one after the other, dry brush and cocktail sticks.
John D