I like to leave the painted track overnight as the enamel needs to be fully dry before ballasting. Not just 'cos it's the dullest job on a model railway and I'm trying to put it off.
If you read a layout article in the model railway press you'll find the phrase "I used N gauge ballast as it's finer than the stuff sold for OO" if the writer is a 4mm scale modeller. It's a little known fact that a law passed in 1952 makes this statement mandatory on pain of transportation to Huddersfield for the magazine editor. It's the same batch of laws that require all model bridges to have a bus on them...
I don't know what the bag said on the granite ballast I used for this bit of track but it's probably on the large side in most people's eyes. Sand is actually a good idea as the tiny grains are far closer the correct size. I have some but couldn't be bothered painting the stuff - this is only a background !
First stage is to carefully pour stone onto sleepers. This will damage the paint if done before it is fully hardened. Then I carefully brush and prod it off the sleeper tops and make a nice neat edge. A big brush and Tamya paint stirrer worked well for this. It's not a quick job though, ballasting probably took as long as making the track ! Don't do it if you have a cold either as one sneeze could undo a lot of effort.
To fix the stone I dribble some watered down PVA (50:50 water:PVA plus a couple of drops of washing up liquid to break down the surface tension) in from the edges of the mound. It's vital to avoid getting any on the sleeper tops as you end up with a shiny surface. A cheap pipette is the ideal tool, I know the cliche is an eye dropper but I don't have one of those to hand.
Once flooded, the ballast is left overnight to dry. I placed some wood on top with weights on it in case the card swelled up. That's why clever people use cork on the layout instead of card.
Now granite ballast, when flooded with PVA, will go a greeny colour. There's nothing you can do about this other than use a different glue. I think Woodland Scenics do something but I've never tried it. Plan B, which I use, is to dry -brush the stones with pale grey paint. Time consuming and bad for the brush, but effective, especially on a small area like this.
Anyway, the glue was left to dry leaving only grassing to do. But for that I wanted to build a Ganja Grasser.