Over the last few months I have tried several methods of ballasting in siding areas. All too often you see layouts where sidings have been ballasted almost to mainline standards when this simply did not happen on the prototype. I have tried various products marketed as either ash or cinder ballast but all are way too course for 3mm scale modelling (and 4mm scale too come to that). This process of trial and error even led me a few months ago to rip up and relay a couple of sidings on the layout.
I then saw an article on Chris Nevard's blog http://nevardmedia.blogspot.co.uk/2011/08/creating-effect-of-ash-ballast.html which has saved the day. I could not get hold of the exact same make of clay that Chris used but found a Korean product called "Mungyo" in my local branch of Hobbycraft.
You can even cover over existing conventional ballast (provided it's not been done to sleeper height).
In fact the old ballast really provides a good key for the plaster.
So thanks for posting that Chris, saved me ripping the darned track up again!
2 weeks ago