Walls – Painting Dark Color

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. :)

Latex paints have a known attribute that allows certain things to persistently come through the paint even though you try applying multiple layers.

I once had a problem of a wall that had a poster glued to it by a previous owner. I removed the poster and apparently the glue residue, however small it was, kept changing the paint I applied over that area.

Another time I was trying to paint a door that had been finished with an oil based product. I had worked diligently to strip and sand off the old finish before painting. Unfortunately the old finish had penetrated the wood to varying degrees in different spots. The paint I applied to the door was not uniform even after four or five coats - hard to remember exactly how many as it was 25 years ago.

The fix for both of these problems is to apply a sealer / primer product like Kilz before applying the final paint. One needs to make sure the surface is properly prepared before applying the sealer / primer. Light sanding can help and washing with a strong solution of hot water and TSP can help.