Hi
Can be pretty annoying eh? We do a lot of yacht refinishing and although I'm not one of the varnishers I've had enough of what you've got for a lifetime. There could be several causes but try these for starters - its good practice anyway.
1) Never shake or stir a can of varnish - you're introducing bubbles. With a closed can rock it around gently for several minutes until its thoroughly mixed. BTW if its a matt/satin varnish you'll have to keep doing this as you go to maintain a consistent finish.
2) The point is to let the varnish flow off the brush, almost of its own accord and just gently spread it even with the brush ie don't push hard or you'll have bubbles galore.
3) Foam brushes are excellent but really are not forgiving - little too much pressure and hey presto bubbles.
4) If you do use foam brushes never use the cheap ones; we always use Jenn-Poly - very tight cell foam, strip of reinforcing plastic in the middle and a nice wooden handle.
5) Find the right balance of load - too little and you get holidays/brush marks, too much and it'll sag or be prone to bubbles.
Just thought, you didn't say which coat you're on or which wood. If its the first coat and the grain is deep its not unusual to get a few bubbles as a little air can get trapped in the grain - first coat should be 50/50 and on that coat brushing the bubbles out doesn't matter.
Hope that helps, getting a high gloss can be a proper 'mare.
