Well my 2 cents.
1) There is a product by "Klean" called after wash, consistency of water, at HD. Pour some into a cup, use 0000 steel wool to dip, then sand off the old varnish. Wipe with paper towels as you go to get off the old stuff. This stuff is a stripper but more mild but still use gloves and do outside. This will minimize any actual wood from being sanded.
2) If you use poly, do as Dave said but USE OIL BASED poly. Also oil-based stain if you chose to alter the color. The water based stuff just does not hold up. I sand with 0000 steel wool between coats. I actually only use one coat because I like some of the grain to stand out, don't like many coats so it is totally slick .
3) Alternative is to use the USC SprayMax 2k clear coat that I and Stan have used on our Bus wheels. Same thing, spray and sand between coats.
4) The lick maneuver, well I do it too.
5) You may be tempted to use some poly used on boats like the Helmsman stuff from Minwax since it holds up better to UV light. The stuff is so hard that it cannot tolerate temp swings, it will just start to crack and peel.