I recently painted my kitchen wall with a metallic paint. I love the paint and the look it gives my wall, except I have brush marks in it. This wall was a new sheet rocked wall that was mudded and sanded. I made sure the wall was smooth before I painted. It suggested that I used a roller with a nap for smooth surface and I did, but I had to use a paint brush on the top of the wall by the trim. When it dried I seen bad brush marks, so I let it dry for 2 days and then sanded it lightly and it seemed as if it was all smooth again. I painted it again but this time I used a small roller after the first coat I still seen brush marks, so I waited a few days and painted it again. It has been 2 weeks and I still have brush marks. I was looking online for a way to fix this, and I found a site that said I should sand it lightly with 120 paper and then prime the wall before I paint it.
Once dry, brush marks will not go away without sanding...sorry.
Once that is done though:
When you paint any given space you should move precisely and quickly. So in this case when you cut out the space in your kitchen (brushing the edges) you should immediately (before the cut work dries) go over it with the roller and get the roller as close to the edge as is reasonable.
Even with good brushes (I agree with the poster who mentions using quality applicators, I was a painter for 10 years or so and they make a ton of difference) you will still see a difference between the sheen on the portions that are brushed and the portions that are rolled but this can be minimized if you roll over the cut portions before they dry as it makes the sheen/texture match.
Edit: Also, if you are using the same paint again there is absolutely no reason to prime after you sand down the brush marks.
Dark colors and metallics do not cover well as they do not have the pigment (titanium dioxide) that blocks out underlying colors .
The use of high quality applicators (brushes and rollers) is critical for ensuring a high quality final finish. Buy a good paintbrush that will deliver an even coating application and you should see those brush marks disappear.
In our "disposable" society, everyone buys cheap brushes with the intent of throwing them out when done, only to suffer from lame results. Get a good brush ($$), clean it thoroughly when you are done, and hang it up to reuse next time, over and over and over. Just set aside time for clean up and it won't be so bad.
If you can borrow, rent, or buy a sprayer, I would highly recommend that for any metallic or pearlescent paints.
The thing about those types of paint is that they have little light-reflective bits in them. As you apply the paint with a brush/roller/pad, the physical orientation of those bits (scientific term) is determined, at least in part, by the direction you're moving, the nap of the applicator, and the amount of pressure applied. In the dried finish, the bits in one brush stroke will reflect light differently than the bits in another brush stroke applied at an angle just a couple of degrees off and you will always be able to see the difference. Sanding won't change the position of the bits -- they're set once the finish dries.
I bought a sprayer after several frustrating attempts to apply metallic paint to a single wall. I paint a lot and don't often use it because I don't like the taping and draping (two hours of prep work, 30 minutes of spraying, 1 hour of clean up...), but it's the only way I've found to get a flawless finish with metallic paint. It's also pretty great for things with a lot of nooks and crannies, e.g., closets with built-in shelves.
If you sand and prime over the part that has brush marks, you could try using a pad trim painter. You won't get brush marks, but you may drive yourself crazy trying to make it look good, since the pad won't look the same as the roller on the rest of the wall.