Generally don't varnish a deck.

Applies to boats, so may apply to a house.

The deck will be needlessly slick underfoot.

Now if you house deck begins rocking and pitching like a boat, you admittedly
will have more serious problems than a potentially slippery deck.

There's no reason that your dry, weathered wood won't accept additional coats
of oil based stain.......just wipe up the excess (as you have done).