If they’re going to be in the sun full time I would put at least 6.  The more the better.  If you have no other use for the varnish it will probably go bad before you need it again anyway.  Be sure and let it cure good before you stick it in the water.  A couple of days anyway.  More would be better.  Put some wax paper around it with a rubber band or tape for an hour or so and it tries to stick it’s not cured.  Use the wax paper because it will come loose.  Paper might stick for good.