I did a similar thing, only misting with IPA, which not only dampened the fine ballast so it didn't float on the diluted PVA, it also helped disperse the PVA solution better than when a drop of washing up liquid is added. I'm happy with the result. I've just cleaned off any ballast that is stuck to the side of the rails and started the tedious job of painting the rails a rust colour. The intention is then to give the track a light spray of sleeper grime, and some light blackening just to tone down that bright fresh even look freshly laid ballast gives. But it could all go wrong as I've only ever sprayed things using Halfords aerosols before... never touched an airbrush