I am often asked how I do the bases for my models at all scales.
When I first started getting into modelling and gaming, I made a lot of mistakes. My first basing attempts were good old PVA glue and Games Workshop sand, with green stuff used to smooth out bumpy metal bases glued to plastic stands.
This was fine but didn't have much variation, and was quite hard to manage on smaller scale areas, with "bald" parts of the base being a problem, or sand glueing to detailed parts of models, needing careful going over with a knife to shift.
I used this as the basis of my first models for 40k, Infinity and Epic, before stumbling on the Vallejo basing materials.
Vallejo make several different colours / consistencies of this product, but generally speaking you should think of it as ground pumice/rock suspended in a wet resinous material. It's a bit like what people have made for themselves using polyfilla and railway ballast for years in the modelling world.
It was this article from DakkaDakka that made me really want to give them a go:
Vallejo Pumice Comparison
I took a couple of pictures of what the material looks like wet on the bases of my models. Above you can see some Infinity 40mm bases. I glued some plastic sprue to act as ruins/girders as they are such large bases and need "filling" to please the eye.
Then I took Grey Pumice, which is my favourite material due to the thick, sculptable texture, and used a Games Workshop plastic Texture spreader to ladle it out and spread it evenly over the bases.
It's a bit tricky to tell how it's going to dry, so making deliberate peaks and troughs is not very intuitive. However, it works much better than any sand or grit on PVA in terms of being able to shore it up to chunky based models, making a smooth transition between the plastic base and where the model is glued, making it look more natural.
The stuff is so thick that you can actually drop little plastic, resin or stone basing parts on top of it, give them a little tap so that they are buried in it and leave them to dry. I use bits of slate to give the surface some variety, and here I've used a couple of Warhammer 40k bits like grenades or ammo clips just to add some interest.
It takes a while to dry, so I usually pop them in the airing cupboard overnight. If you want to do something really thick, you can wait until it dries then put another layer on top, but once you get the hang of it, you probably won't need to do that on normal bases.
I have used it on my SAGA, EPIC, Infinity, Warmaster, my terrain, and I'm really glad I did!