I had tiny bubbles in my dried poly. I looked through a jewelers magnifying loupe. Yup, bubbles. Thought it might be sand or dirt, or dust. They were half in and half out, but I could neither brush them out or sand them out. I am sure that there are scientific words, and a possible explanation and solution. I don't have them. I could not block sand the bubble in half, without destroying the surrounding area. And subsequent coats would have same anyways. It would have left concave half pitts if sanding was possible. so there they are. Too deep to ignore, and not raised enough to even half fix!! Don't know what I did wrong or not right. it was inside of drawers, and the right finish, so what are you gonna do.
I personally think the only solution is not to get them in the first place. I used a new purdy brush, probably coated the brush with solvent and ficked off. May have forgot, not sure. Don't think I thinned first or any coat (3) because the can does not mention it. It was spar, not hard type cause I had it already.
I can only guess that a little thinner may have been the solution. And maybe not in the order of 10% which may be too much. Could be as low as 1%. I think experimentation is in order. I think this applies to all coats, and don't know if more thinner on the first coat as primer is needed, whether or not the can says so. NB i did poly outdoors once in high temp and very high humidity, and the water got right into the finish and it went white, like milk paint. Next tiem, snaded down, not to wood, and it was gone, gone. I think you could tell you had sanded it all out. Yes it was that dramatic.
For arguments sake, has anybody done a descent job of poly with w/ a brush, or foam without bubbles. Simple ya, na is sufficient. If speciically so, was it thinner, and how much?