What I found with sand on bases is if you paint the base and then just stick sand on then i will inevitably fall off.
So what I do is stick the sand first then spray the model then paint the model, then paint the base. Because you have the PVA glue holding it and the primer and then the paint.
The colours that I use to paint my bases are : Calthan Brown(basecoat), Badab black(Wash entire base), Tausept ochre(first drybrush) then Iyanden Darksun(Final Highlight).
Hope this helps your sand problem
I've had no problems with sand, but then I'd suggest painting the sand, not just using it bare.
like for me I glue it on with PVA, once dry spray it black, then either spray or paint it grey (for urban, grey can be replaced for whatever color though for whatever environment), sometimes wash it, and then purity seal it when the whole model gets done, its rare then that it ever falls off unless you really pick at it or bash it around.
Dullcoat will also protect the model and stabilize the sand. Testors is good, but don't use it when it's humid. Another option is acrylic floor wax, it's sold as "Future" here in the US. Apply with an old brush you're throwing out.
Try using wood glue instead of pva to glue the sand to the base. I paint and seal the figure before coating the top of the base in wood glue and then sprinkle the sand over the glue. I've never had problems with the sand coming off later, even when the figures have been dropped. Be careful about how thick a layer of glue you put on, though, since wood glue doesn't dry clear like pva does.
I use a mix of sand, flock, and small gravel (all sold by GW)- I put a blob of pva on the base, spread it around with an old brush, dump said base in to the sand/flock/gravel mixture (after having placed lumps of blue tac on the base where my model will eventually stand so the mix and glue don't prevent the plastic cement from bonding).
Give the base a lick of Devlan Mud wash, and when that's dried I give it a very, very short burst of purity seal. In 9 months sand has yet to come off.
apply glue (pva usually). Apply sand.
WHEN DRY: Apply a thinned coat of pva (1:3 or 1:4) and water with a drop of dishwashing liquid (brand is irrelevant as is eco-safeness, it's a surfactant that lowers surface tension).
The surfactant allows the glue wash to flow around EVERY grain, not just flow over the top of the surface, leaving the subsurface dry. When it dries, it locks each grain to each other.
Then I paint it, giving it another layer.
After the model is attached, the entire thing is then varnished.
I've got models going back 20 years done this way and they still have their original basing materials on them.
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