I have a wall that is 8 feet tall and about 20 feet wide with a strip of chair rail that runs the length of the wall. I used Sherwin Williams satin white paint. I gave the wall a light sanding with fine sand paper before I painted. I cut in the top, bottom, and the chair rail with a brush. Then rolled on the paint with a Purdy roller. The wall is now full of streaks (even after a second coat). I used the "W" technique (always worked fine for flat paint). I plan to repaint it again with the same paint and hopefully do better. I am reading to roll top to bottom from some sources while others say the "W" technique. What is the best way to avoid streaking with satin paint?
By streaking in satin paint I assume you mean flashing. Flashing is when you can see differences in sheen at certain light angles that looks like you just went over some areas with wet paint and other parts look dryer. It's more noticeable with higher sheen paints and oblique lighting like you might find from a nearby window on an adjacent wall. Does that sound like what you mean by streaks?
Flashing was something I always had a problem with. One of the things that leads to flashing is I believe an uneven coat. When I keep the roller consistently loaded I don't get flashing.
The W technique never worked for me and I don't think I've seen a professional painter roll on paint that way. What I've always seen them do is roll up and down the wall just the width of the roller. Then reloading and doing the next strip.
What I do is load the roller, roll off the excess in the pan, start in the middle of the wall, roll up, then down only 3 or 4 times. Reload, roll of excess, overlap the next section a bit. When I'm done rolling the next stripe I will roll back over the previous area slightly to even out any roller marks. Also I stopped trying to squeeze the most I can out of the roller when I'm almost done painting and I'm almost out of paint and too lazy to deal with opening another can. Keeping a consistent level of paint on the roller, not pressing it too hard and just letting the paint do the work has helped a lot.
If you move down the wall and notice you may have missed a spot on an earlier stripe, just leave it. Touch it up after it has dried enough for the next coat.
I also have one of those Wagner power rollers that pumps paint into the roller which makes it easy to keep the roller consistently full of paint. When I use that I've never had flashing issues.
Lately I've started using Benjamin Moore Aura which is really good when it comes to flashing. I wasn't sure if it was just the paint or my technique until a few months ago when I had to do some painting with a non Aura eggshell finish paint and I didn't get any flashing even though I did it fairly rushed using a roller and tray. The other paint is a good paint but I've had trouble with flashing in the past. Especially on ceilings.
Assuming you are indeed applying enough paint (if the label says 400 sqft/gal that is what you should cover per coat), regardless of the application technique chosen ("W" or "V" methods work fine) it is very important to maintain a "wet edge".
This means preparing your surface, equipment, materials, and work area such that you can work continuously and quickly enough so that any overlapped areas are still wet. You then blend the overlapping zones with a few feathering roller strokes. If you do not maintain a wet edge you will end up with areas/stripes that essentially have 2 coats at the point of overlap. This can result in the appearance of "streaks".
You should be able to remedy the issue (regardless of root cause) with a nice second coat over the whole area.