In the past I would assemble my model, glue it to a base, prime it, and then paint it. Once it was all the way painted I would put down a layer of glue and the add ballast, paint the ballast, and seal the model. I stopped doing this for a few reasons.
The first reason was that it was messy. I would always get paint on my model when I was painting the ballast I added. I would always paint the ballast black so I ended up getting black on my model and have to redo the section or just leave it and have the model not look as sharp.
The second reason that I stopped doing it this way is because it covered up the bottom of the models shoes. It looked like they were stuck down in the ground. This may be ideal for a few larger models, but for rank and file troops it just didn't look right.
Here is how I do it now. The first thing I do is prepare the base. I take a round base and trim off any sprue. I then add a layer of glue and dip it into the ballast. I will then let the base dry overnight and take my finger and roughly try to brush off any loose pieces of ballast.
Once I have done that I take a watered down mix of PVA glue and give the base a good coat. This will help keep any of the ballast from falling off in the future. Next I prime the base black. This keeps me from having to put a base coat on the base later.
After the primer dries I am able to glue my primed model on and continue painting as normal.
Below are a few pictures I took of my finished Chaos Spawn bases.
