Having applied sand, I leave them to dry a little, then go back and scrape inside the recesses with a sharp knife to remove the inevitable overspill. It's time consuming and messy.
Here's an example of the sand finish, these aren't the sabot bases, but markers I did for wounded leaders. Sabot bases from Supreme Littleness, skulls were in my accessories box.
So as I looked at the finished result after a couple of hours.. it occurred to me I could get virtual identical results if I could buy a 'top' for the base, cut from medium Sandpaper to the same spec as the base, and just stick that on.
So I can't believe no-one has suggested this? If Martin (Warbases) is watching... personally, I think it's genius, and a real timesaver.