It sat for about two weeks. I really did a job going over it. Sanded from 160 at the raised areas) through 320, then 0000 wool. Really, really smooth. Rub a hand across and it was smooth as glass. Perfect.
Had run the fan with the ultra fine filter on it, no visible dust in the air. Washed down the area, had vacuumed 3x. Tack clothed multiple times. Heated the space. Got a new chinex soft 3" brush. Leveled the surface with a long level to perfect so that the urethane couldn't flow.
Thinned the (strained) urethane with naphtha this time around 20% so it would dry faster. Applied it.
I did see a few more, larger bubbles, but they popped fast.
But by the time I got to the one end I could see nibs on the other. Ugh. Plus it wasn't flat. I think the naphtha makes it run more than mineral spirits.
Now what?
Per the post immediately above, it will never be perfect, these nibs will always show up. Not sure if bamboo is worse in this regards?
I'm thinking of sanding with 400 as soon as I can, brushing one more layer on, uneven or not, and then when that dries for a week or two, move to 4f pumice and rotten stone.