outdoor steps or ramps: I've got them for my shed, my deck, and getting into the sauna. They can get treacherous with time/between cleanings, algae, or just in the rain. Here's a no-brainer solution.
Step 1: The Whole Thing.
outdoor marine varnish:
1) one thin coat to seal the wood, let dry.
2) in a cup mix some play sand with some of the marine varnish, put on the second coat, let dry
3) third coat on top.
You'll never slip again.