I think part of the problem is I'm using a very good (and expensive) 3" natural bristle brush that I've been using to apply Minwax semigloss polyurethane (not water-based), multiple coats, on several large tables over several months and have been keeping the brush (the bristles part) immersed in terpentine (in a covered glass container) with the handle suspended via a wire through a hole (I drilled) in the handle so the bristles come nowhere near the bottom. This has worked well for nearly several months and saves me much time not having to clean the brush after each application and after each table. I simply leave the brush immersed between uses. However, it may be that the brush is now reaching the end of its functional utility. Therefore, this morning I tried a 2.5" brand new high quality synthetic bristle brush, and though it also left some small bubbles, it did not leave as many, and for those it did leave I gently swept the brush over the bubbles/wet polyurethane (on the table) at an acute angle applying no pressure at all (relying only on the brush weight) across the entire length of the grain on the table, and this eliminated nearly all of the small bubbles and delivered an even coat like always. I will try this approach again with the next coat.