There are many different non-skid materials that are available to add to coatings to create a more slip-resistant surface. When applying a non-skid surface to varnish, most people want to create a good footing that is not too rough for bare feet, but does not have a sense of opacity.
One of the most popular non-skid materials for varnish is the Epifanes Non-Skid Beads, which come in one particle size. These beads are made of polypropylene and are designed to absorb the varnish, boasting the best transparency. AwlGrip and Quantum paints offer their version of beads in multiple particle-size options. These products are not designed to absorb the varnish, but due to their cloudy/clear nature, they have reduced opacity when used with varnish. SoftSand offers their rubber beads in multiple sizes. These beads are designed to be less aggressive to the touch, but due to their color, you may see the particles. Interlux's non-skid material is very easy on feet, but may not offer enough traction for some applications.
IMPORTANT! It is strongly recommended that you never use sand or any extremely hard or sharp materials for non-skid additives. These materials can be very unsafe if someone falls on them, and extremely harsh to the touch.