Hey Nick, Well done having a go at this method.
My personal opinion is that it looks more muddy than Sandy - so unless you're going for a Mimban look, it probably needs to be a little more subtle.
Speckled, rather than painted, and maybe a little "less" in the crevases.
If it was real sand, most of that would realistically fall off unless it was permanently wet - which it isn't in the desert.
Hope that helps mate.