Put seven Garden Railway'ers in a room and you'll have fifteen opinions about ballast.
There are a great many who simply don't ballast their lines and I can quite see why. The function of ballast on 12":ft lines is to provide a firm 'gripping' base for the sleeper to sit into. In general, for a garden line, the function is decorative. That said, there are those who apply the prototypical way, and do just sit their track on a suitable (I guess fairly deep) layer of ballast. This may be easier with LGB track which has a huge profile and thus doesn't deviate much, whereas I'm using flexible peco G45 track which is somewhat more inclined to flex - not unreasonably.
If you decide to ballast, there are two questions.
This is where I started - and made all the mistakes, so listen carefully. The 'obvious' choice seemed to be fine gravel from the garden centre down the road, and dribbling on PVA in much the same way as I'd ballasted a few miles of OO.
Two basic flaws with this - and one messy consequence. One, the gravel was (a) a bit big - scale it up and even quite small stones are slightly larger than scale footballs, and (b) it was rounded - which makes the football comparison even more pertinent.
Study real lines and you'll notice they aren't ballasted with footballs.
I persevered with this for a bit, as you do, picking out the larger pieces. The photo to the right shows it at it's best - with the larger pieces either picked off or blown/washed away. Ths introduces the second issue. I'd held the gravel down with good old PVA. It had worked before... indoors. Unfortunately this wasn't waterproof PVA and while it worked for a few months, nothing of this ballast is still attached except by gravity.
However, the big mistake I'd made was relying on a combination of 'no nails' adhesive and the ballast/PVA to hold the track to the trackbed. No Nails proved positively useless at this, and the PVA equally so, so the track moved with temperature fluctuations etc. No intrinsic problem with this especially as I'd allow expansion gaps etc. However, gravel got under the sleepers and the track started lifting...
Time to screw the track down. Much whinging and drilling of concrete, but it ain't moving no more. Use brass screws. Real brass - not Q&B shinies. All that glisters is probably a nanometer of brass on insta-rust steel. Use a magnet.
Two big changes here. One was to source a much finer grit that was actually gritty. The other to mix it with cement. There were finer grits available that were for showing off your exotic alpines - bagged small, priced big. However, I did find a cheap supplied of recycled material (I like recycling) in the EcoAggs range from another garden centre. This was sold as potting grit and is perfect texture if a bit small. Unfortunately it's white. Now, white ballast is perfectly prototypical in some areas of the UK, but it does glow a bit. I'm hoping it will (and I'm encouraging it to) mature after a bit, especially with a live steamer dripping oil onto it.
On more recent sections I've mixed in some fine 'alpine' grit and some sand, and this helps both the size and colour a bit.
The grit/sand/cement/... mixture is mixed, and laid, dry, then brushed in with an old paintbrush or similar and finally gently sprayed with water - just a fine mist.
So far, this has held firm over the winter.