Spraying varnish is not usually a good idea. Quick drying products such as lacquer, shellac, and w/b finish, dry as atomized droplets then fall as dust.

Slow drying products like varnish (and Waterlox is a varnish) the atomized droplets land wet and dry where they land.

Further, if the varnish sets up in your gun, you will have a heck of a time getting it cleaned out. The fast drying products are easily removed with their respective solvents, and w/b with acetone.

get a black bristle "China Bristle" brush for oil based product. NEVER let it touch water. Not even one drop. Rinse it in mineral spirits, then rinse again in clean mineral spirits. Spin or shake out excess and hang it back in its original package to dry. I don't recommend soaking it in mineral spirits overnight because the resins in the varnish precipatate into the solvent and settle at the bottom. This is like a thin layer of scum the next day, and you don't want that on your brush. Oil varnishes could be sprayed, but you aren't really accomplishing anything other than alot of stinky vapors, a mess in your spray equipment, and runs and drips all over your project. Like Bob said, fast drying finished perform well in spray equipment, with one or two application coats. Slow drying oil varnishes need several thin coats to build a film finish. Put on a thick coat of oil varnish and it will run and drip for sure.