How to clean brush after polyurethane

I've had some success with recovering a gummed-up brush using a paint brush comb. Soften the bristles by soaking them in thinner for a while, then use the comb to brush out loosened paint; repeat, probably several times. If the brush is really bad, you may just be better off buying a new brush.

It's hard to get a brush completely clean, so to help prevent this, I have a large glass jar with some mineral spirits in the bottom. I hang the brush from the top of the jar with wires so that the bristles are submerged (but not resting on the bottom). Seal up the jar and the brush is OK to use next time.

When you say you can see the brush marks, do you mean that they're actually irregularly surfaced? I mean - are we talking just a visual effect or an actual difference in the depth of the paint?

If the latter, do a skim coat with lightweight joint compound and when it dries, either lightly sand OR smooth with a large, slightly damp sponge, then apply primer.

As for your own brush strokes - orient the roller so the roller bar leads your motion. If you're moving right, put the bar on the right. If you're moving left, put the bar on the left. The most pressure will be on the roller bar side, so keeping that side on the "front" of your motion will mean the lighter touch side (the back) will smooth out the uneven strokes as you pass over it again.

Now if its not a real issue with texture, just that your seams are obvious - then just prime it again with a good HEAVY primer, using the proper brush techniques, and you'll be fine. :)