The easiest way to avoid it, is as the other poster mentioned, use
wipe-on. Just thin it 50% with mineral spirits.
If you us a brush, use these steps to avoid bubbles, rope lines, etc.
1. Don't pickup bubbles into the brush. If you have bubbles on top of
the can, push them aside with a stick before dipping the brush.
2. Use "tipping off". This is a technique where the final stroke of the
brush is made with the brush exactly perpendicular (90 degrees) to the
surface and just lightly stroking the very tip of the brush across the
surface to flatten and scrape any bubbles away.