After a lot of time spent getting the layout running, then it is covered with glue and grit and afterwards still ,hopefully, works!
These are the ingredients. Ballast- in this case 'N' Gauge granite, Mist spray bottle, Eye Dropper, soft brush and glue.
The spray bottle has a couple of drops of Washing up liquid added- breaks surface tension and helps the glue flow around the ballast without it lumping.
The glue I use is a PVA, sold cheaply in Poundland for mmm- a Pound! This is watered down with about 50% water and a couple of drops of washing up liquid to ease the surface tension- not too much or the whole lot disappears under a sea of bubbles.