I use ‘Sand’, quite often with my buttercream. I used crushed up graham crackers. You can use any kind of cookie, in that shade, run through a food processor and ground fine. Sometimes I will add a little brown sugar, for sweetness and glitter.
Usually, I have seen the ‘sand’ adhered to buttercream cakes. I think if you want it to stick to a fondant cake you should try to use some clear piping gel. Spread the piping gel in the fondant, and then using your hands, press the crumbs to the gel.
Personally, i would try this, and then let it dry overnight to make sure the crumbs will stick.
Types of cookies I have used for ‘sand’, ’Nilla Wafers, Windmill Cookies, Plain graham crackers, A mixture of nilla wafers and graham crackers and once when I was in a bind, I ground up a couple of waffle ice cream cones!
I hope this helps. I live at the beach and have many request for beach wedding cakes in which I use sand, you can check them out in my photos.
As a side note; If you are going to adhere anything else beside the sand such as a flower or a seashell, do that first because you will never get anything to stick to the sand, nor will you be able to write on the sand.
I adhere my seashells first, with some buttercream and then sprinkle ‘sand’, around the shells or flowers. I then use a food only paint brush, and sweep the crumbs off the seashells.
I hope this helps!