In addition to what Rob Macks has discussed let me suggest a foam ( Fen) brush. I have been varnishing wood for several years and I tried high quality bristle ( China Pig's? ) but always ended up with contamination in the brush itself. No matter how much I cleaned I couldn't get it right. I quit using those brushes and have used foam since; it doesn't flow as perfectly but you can't tell the difference in the finish and there is nothing to clean up ( or mess up your next coat)
Only change from Rob is I use 320 grit between coats, just to scatch the surface (and to identify any drips/drabs from the last coat).
I have never had to thin Captain's; it seems always ready from the can.
Alex Warren