The next stage is to fill in the rest of the bases with finer scatter. I have a tub of various grades of sand and grit plus small stones that I use for this.
First you lay down some PVA in the empty areas and between the rocks and then "sprinkle". Once it is dry you then get out some more PVA, water it down (50/50) and give it a top coat which binds all the sand. I also coat the bark and stones - provides a better painting surface - but leave twigs "au natural".
You then leave them for 6-12 hours to dry completely. When that is done you should have completely bound surface for painting.
My first step is to give everything but the twigs a coat of black primer and then leave it to dry.
The next stage is to start painting your base.