More ballasting. More ash.
If anyone suggests ash ballast is a good idea for your model railway - slap them. It's not, it's horrible stuff to work with.
My initial experiments showed that if you just lay ash, pour on the watered down PVA and leave to dry, the ballast shrinks as it dries leaving gaps around the sleepers. To fix this, I mix in powdered household wall filler.
Of course this stuff doesn't spread easily like small stones. It sticks to everything, coats the sleeper tops, won't move around properly. Horrid. Horrid. Horrid.
Next, I find that the PVA doesn't run through the denser mix like it does through granite. No, it puckers up at the touch of liquid, even if said liquid has the magic surface-tension busting washing up juice in it. Patting it back helps, the end of a 3 inch paintbrush being particularly good for this job.
After all this, the colour isn't right. Putting black powder paint in the PVA mix helps, but I still need to wash it over with some more greys and browns.
Truth is, granite would be a heck of a lot easier and I bet most modellers, having grown up looking at model railways rather than real railways, probably wouldn't notice. Still, I know it's right and once the hassle of the work is forgotten, I will be able to bore show visitors with my tales, so I suppose that makes it all right then.
And the greaseproof paper in the baseboard joint? Hopefully, it will stop the boards sticking together.