Our southern oak has a lot of open grain so it is hard to tell when the stain is dry below the surface of the wood. If you use a foam brush, brushing too fast will cause bubbles. So it will spread easier and bubbles will be minimized. Unless you have done lots of air scrubbing in your "finish room", there is always dust in the air. That rules out applying polyurethane in my workshop or applying polyurethane outside. First, make sure the finish has enough solvent in it.

Theories abound … NEVER SHAKE POLYURETHANE, and you will avoid most air bubbles. Just look at the instructions on the back of the container… polyurethane must be applied in a dust free environment. Removing bumps in polyurethane finish. Yes, very reasonable, lots of people do just that :-) You can wet-sand with diluted poly (wear gloves!) The usual cause of the air bubble entrapment is from shaking the can of polyurethane prior to using it.

Thinning the finish will make it flow out better so brush marks will level and air bubbles will have enough time to burst before the finish sets up. Question I have a large conference table that I built using MDF and 22 ml paper-backed maple veneer. Avoiding Bubbles with Spar Varnish Brush technique is key. I have had good results with both varnish and polyurethane and foam brushes. Other than this, polyurethane has a peculiar problem associated with its hardener - the hardener being isocynate, it reacts with moisture and forms carbon dioxide as byproduct. When I run my hand over it I feel small bumps." Not sure if this caused the next problem. Resist the temptation to shake the polyurethane prior to use. Avoiding blemishes in the finish--like bubbles--requires careful preparation of the surface and attention to detail when mixing the resin with the hardener. None are "waterproof". So it will spread easier and bubbles will be minimized. Stain that has not dried before the first coat of finish can cause bubbles.

This is probably something best left to the first coat, similar to how some people sand in the first application of BLO to help fill pores. Brush out bubbles in a fresh polyurethane finish. My experience is that the bubbles will knock off but you will be left with a small crater, which is less noticable but still there. It's not an uncommon problem, especially when applying polyurethane as the finishing coats. After putting two coats of spray-on polycrylic, only waited 48 hours when we were supposed to wait 72. Polyurethanes trap air bubbles if they are introduced, and are gooey enough that they don't allow them to pop and lay flat, even after being brushed on. Each time I've tried to be more careful about creating bubbles, but the finish was still riddled with bubbles. Most every painter has experienced the problems of bubbles in polyurethane finishes. There're lots of oil alternatives and three primary resin alternatives: polyurethane being one of them. When the finish hardens, you have to sand them out and apply a new finish.