[Therion] Tricks for drawing boulders on sand area without sand inside the boulders?

I am wondering if I am missing a trick here. I am trying to draw some boulders in specific positions in an area of sand. The boulders are drawn with rock-edge line type (is that correct?). But as you can see on the PDF, the sand area fill puts sand inside the boulders. Is there an easy way to avoid this? Or do I have to carefully define my fill area around the outsides of the boulders only? Footleg