Instead of non-skid tape, these treads have been sanded. Sand is more durable than anti-skid tape which tends to wear unevenly and look scruffy in highly trafficked areas. Unlike anti-skid tape that sometimes lifts at the edges, there’s no chance of that happening when using sand.
1/ Abrade the entire tread.
2/ Mask around the area to be made anti-skid.
3/ Varnish the masked area then sprinkle heavily with sand – new blasting sand is best.
4/ Allow to dry 24 hrs.
5/ Remove tape and brush away loose sand.
6/ Completely varnish over the whole tread, stippling well into sanded area.
Tip. Pour a small amount of varnish into a can or container to use, so as not to contaminate the good stuff with sand.
For more good ideas: Practical Boat Bits and Tips by Jude Binder