I can't add much to what has already been said... Most varnishes do work better with a brush made for them. The better brushes, like Purdy brand do cost a lot, but they last for years and sure lay down a nice smooth coat. Just clean them good with spirits then wash them with dish soap.
OR... do what most sign painters do. Wash them in spirits then wet them with thin motor oil! Squeese out the excess, shape the tip to a wedge and put them back in their factory sleeve or wrap in newspaper. When you go to use it again slosh it in some spirits and shake it out... then paint. The motor oil will keep paint from ever cloging up the hilt of the brush and will never dry out in storage. I've got 20 year old brushes I still use.
As for bubbles... Thin the first coat until the bubbles will self level & pop. For large flat areas some people like to use a pad coater instead of a brush. The don't have much tendancy to create bubbles. Thinning till you get a wiping varnish is good too... Just takes more coats to build your finish.
Oh... like he said above, never shake varnish; just stir it if you have to mix thinner into it. When you are done for the day, week of whatever... Store the can upside down! It will never skin over the top that way.
Of course... I'm spoiled. I much prefer to just spray most finishes. No bubble problems at all.
Friend ~ Brick