Why could it be that when I polyurethane several items with the same poly and the same brush (and all items have similar texture) some of them come out smooth and some come out bubbly?

Are you stirring it creating bubbles before you use it?

Is there anything different (ink/paper/texture) about the pictures you’re using on that one plaque?

Otherwise my best guesses:

The wood needs to be sanded (super fine) and wiped again
There’s trapped air between the pictures and the wood
Something about this plaque or these pictures is different than the others (wood type, grain)

Maybe unseen imperfections or fingerprints in the object.

I have not stirred this poly in a long time (months). I know not to stir the poly at all before doing this. The pictures are all printed off the internet, so they’re all from the same printer.

Could be the temperature of the wood, the density of the wood, different make of wood, or moisture in the wood.

I bet @JP is onto something @jca. Maybe the wood was treated differently.

hopefully there is a quality brush being used. A process called “tipping” helps when applying finishes. It’s when the very end of the brush is lightly pulled along the surface to smooth out bubbles as well as the coat of whatever the finish is. There is a very short time window that this can be done successfully after the time when it is applied. The clock is ticking fast as soon as you begin.

I agree, it is the moisture content of what you are painting. Once I had a concrete floor painted with a poly type paint. The pad had been laid for about 2 or 3 weeks but hadn’t cured properly and it bubbled and was a hell of a mess. Maybe you could try microwaving the object before you paint.

I always stir before I use polyurethane, just don’t shake it and cause bubbles. If you don’t stir it, it wouldn’t be mixed completely and that could cause drying problems.