The kit arrived this morning - contents are a bottle of oily black paint (brush or airbrush) and a bag of what looks like grey talcum powder. I also ordered a bottle of their grimy track paint at the same time.
Sorry, no time to film an ‘unboxing video’.

It appears that the powder is brushed into the existing ballast to ‘clog up’ the gaps in the granules. Fixed in place with dilute PVA then painted. That’s the theory, practical tests to follow in due course.

Hoping to recreate something like this:
