use a table saw to re-cut the grove...just a hair deeper and wider than the original. Then go ahead and strip the rest of the varnish.

But I'd never varnish a swim ladder step to begin with. What could be slipperier than a wet bare foot on a varnished piece of teak? Of course you could use some non-skid grit in your varnish. Think of what fun that would be to sand off. Good luck, Doug.

Say! Aren't you a lawyer?