That is my major concern; I think I would avoid Epoxy for such a permanent application, especially with the likelihood of water, snow, ice, etc. sitting atop the surface. You could possibly use a Polyester Flocoat; they're quite viscous so difficult to lay perfectly evenly but we used to (my place of work prior to EC) use them with a carborundum grit to give a non-slip finish to boat decks. Alternatively a gravel bonding/driveway resin may be suitable.
If you could apply it evenly enough, you could possibly then apply the sand/grit all over this surface to achieve the sandstone look.