Paint is easy because it doesn’t have to go on perfectly; brush marks and other minor flaws are expected and accepted. Varnishes are easy because they dry slowly, so there’s plenty of time to spread them out evenly and get the brush strokes lined up with the grain.
Polyurethane varnish is the finish most widely brushed by amateur woodworkers, and it is the finish that is almost always used in instructions on how to brush. (See “The 7 Myths of Polyurethane” in the finishing section at popularwoodworking.com.) There are countless articles, several videos and even segments of television ads that describe or demonstrate how to brush varnish.
For a reason I don’t understand, almost all of these instructions suggest or show brushing slowly, several at the incredibly slow speed of eight seconds per foot. (Try it; I’ll bet you can’t brush this slow the first time).