A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink (and hence, polymerize) by the action of oxygen (not through the evaporation of water or other solvents). Drying oils are a key component of oil paint and some varnishes.
Rubbing with the grain may pull the filler back out of the pores. Get the wood as clean as possible and let the filler cure for three days before sanding. No matter how well you wiped, there will be some dried residue on the surface. Remove it by carefully sanding with 220 or 320 grit paper until there is filler only in the pores and not on the wood between them.
Spar varnish is perfect for outdoor projects and for raw wood used for exterior doors and trim on rustic homes. In addition to protecting the wood, it also provides natural ultraviolet light protection. Spar varnish is often used on items that will be near or on the water, like a wood boat, decks, beach chairs, etc. Apply using a natural-bristle brush.
Commercially, the term "paint thinner" is rather broad, and can cover many different solvents like mineral spirits, naptha, even turpentine. Most, if not all, commercial paint thinners are mixtures of several solvents which are bound to change from brand to brand and even batch to batch.
One of the main differences between varnish and lacquer is that the latter is much quicker to dry. Lacquer, due to its evaporating solvents, will always dry on a surface much quicker than varnish. As lacquer is naturally glossier than varnish, one coat is usually sufficient, while varnish may require several coats.