The gangway ladder was sanded to bare wood, coated with two coats of Le Tonkinois Marine Linseed Oil Varnish, the steps were taped off, the taped-off sections of the steps were liberally coated with varnish, and then sprinkled with grit sized 18/40 ground walnut shells.
Once the vanished dried, the excess shells were blown off and the remaining shells were coated with four coats of vanish,
The resulting non-skid is aggressive (what I was looking for), but is still not uncomfortable to stand on with bare feet. Besides looking nice, an advantage of walnut shell is they can be easily sanded off if the ladder needs to be refinished again.
