Varnish would never ever hold up outside. There's 0 UV protection. Just setting a damp glass down on a varnished table top will leave a ring.
Even a marine grade poly would not hold up for long on a flat top like that.
The only one I've worked with that would hold up the longest is Bristal Finish if your willing to pay the high cost for the materials.
Long ago I used to live and work on boats. When you see an antiques boat with that high gloss smooth as glass wood work that's because there's about 7 layers of very expencive finish on there applyed with a natural hair brush.