PVA glue with a half water mix and a drop of fairy liquid to help the mixture flow. It dries clear and invisible. Do a test sample and see for yourself. I use it when creating wagon loads with real coal and rock. Put the scatter (or wagon load) in place first and drop the glue mixture on it. For precise application I use a dropper from a chemist which costs about 50p. The mixture gets soaked up like a sponge. Leave for 24 hours before removing the loose stuff.
Happy modelling
Gary
Happy modelling
Gary