FWIW, my cousins, who restore wood boats for a living, gave me the following advice when they helped to re-do our cabin sole (they did all the cutting, my wife and i did all the varnishing):
1. Dilute the varnish with turpentine using a 80:20 ratio (80 parts varnish, 20 parts turpentine), this helps it flow better
2. Use a very low nap roller brush to apply (Don Casey's book doesn't mention this, but it worked like a charm). Roll very slowly.
3. The recommendation for the floors was 3 coats of high gloss epiphanes followed by 3 coats of rubbed-effect epiphanes
4. Between coats, we sanded with 320 grit foam-backed sandpaper, and then followed that with tack cloth. After the tack cloth, we rubbed down the wood with a damp/dry paper shop towel that had been soaked in turpentine
Here are the results (with one old floor board for comparison):