On Fri, 15 Jun 2007 14:51:35 -0700, wrote:

What varnish should I use for (treated softwood) wood that stays wet
more or less all day? The first stuff I tried failed very quickly

Use the right timber species.

The right timber doesn't need to be treated. Treated timbers won't be
the right species, QED.

Varnish won't make a blind bit of difference.

If it really is continuously wet, that's a let better than alternating
wet and dry.