I am repainting the interior of my home. I've been frustrated
in my attempts to get a smooth finish on the first 3 closet and room doors.
I've tried brushing and airless spraying. Brushing left brush marks!
The Airless sprayer leaves in some cases long drips along my spray
stroke path,
and sometimes a somewhat rough surface (perhaps too thin a coat).
I am using alkyd enamel paint, semi-gloss. The original color
is beige. The new color is medium-bright yellow.
The brushes were sold to us by Sherwin Williams as first-quality (and
I believe it). The Airless Sprayer is a  Wagner 2200, 2 speed.
I sanded the doors, and primed a few spots. 
I brushed the doors lying horizontally on sawhorses.
After the brushing failed I sanded the doors smooth before I tried the
airless spraying.
I sprayed the doors leaning against a wall in my garage.
I have 11 more doors to go.
I don't think the problem is contamination after the paint was laid
down.
How should I prep and paint these doors to get a smooth finish?
Should I use a roller? Is the Wagner sprayer a poor choice?
(the builders beige finish is very smooth).

Thanks.

I've found a roller to be very effective in achieving a very smooth
finish that is pretty much indistinguishable from a factory-applied
finish.

We recently changed our interior doors, and I had the same process
issues. I wanted the finish to be smooth, but I wasn't sure how to
achieve it. The folks at Lowe's told me to use rollers. My concern
with that was that they are those 6 panel doors, and I didn't know how
I would get into the crevices, etc.

I happened to own a Wagner Sprayer as well, so I thought I'd give it a
shot. Since we replaced our doors, not painted over, I kept one of the
old doors in the garage with me at all times, to 'practice' on, and to
restart spraying when I had to stop for some reason. I found that the
sprayer would spit at the beginning of the stream, but usually not in
the middle.

When it does spit in the middle, I had a sponge paint brush (the kind
used for crafting) with me, in my free hand, and just swept the sponge
brush over the spit marks. Then I would respray.

The doors ended up looking beautiful. We've been doing some
renovations in the house, and people have even commented on them.

Here are some other suggestions:

Cover everything else in your garage and lay a tarp on the ground. The
sprayer gets paint EVERYWHERE, a fine mist. I'm still sweeping it out
of my garage.

Wear a face mask. They are horribly uncomfortable, but if you don't,
you'll be blowing yellow paint out of your nose for quite a long time.
And who's to say how far down in your system the paint went?

Be patient. I set up 4 or 5 doors at a time, leaning against cinder
blocks (with a towel against the cinder block so as not to mark the
door). I would let it drive overnight, flip the doors, then do the
other side.

Make sure the paint is the proper thickness/thinness. Use the V shaped
thing that the Wagner sprayer comes with to check the viscosity of the
paint. That makes a HUGE difference.

See if you can find an old piece of plywood or something to use as
your starting ground. Start spraying on it, then sweep over to the
doors. If I hadn't used this technique, I probably would still have
doors out in my garage, and none in the house. I completely understand
your frustration.

I would stick with the sprayer, once you get your technique down, it's
easy, and quite professional looking. (I won't mention that the
sprayer is heavy, and your arm probably hurts!)

Good luck!

You could perhaps try thinning the paint a little (with mineral
turpentine or "thinners") - not more than about 10% I'd say.  If it is
very hot where you are working, the paint may be setting before it has
a chance to flatten out.  Delaying drying by thinning it should help.

The top professionals use extraction fans to clear out all dust while
the paint is drying.

HI! QFENNESSEY-GA. HERE`S A NEAT LITTLE TRICK I GOT FROM A FRIEND WHO
IS A PFOFESSIONAL PAINTER, I TRIED IT ON MY LITTLE BOAT AND IT CAME UP
A TREAT. USE A FULL OR SEMI GLOSS OIL BASED PAINT, GOOD QUALITY. HAND
SANDPAPER THE EXISTING PAINT TO KEY IN THE NEW. GIVE ONE COAT OF
EITHER OIL BASED UNDERCOAT OR WATER BASED FLAT ACRYLIC. IF USING THE
ACRYLIC, GIVE AT LEAST TWO WHOLE DAYS TO DRY AND CURE. AT LEAST 24
HOURS FOR THE OIL BASED U/COAT. THEN APPLY FIRST COAT OF GLOSS OR SEMI
GLOSS FINISH, HAVING THE PAINT AT "NORMAL" CONSISTENCY, SO IT FLOWS
BUT NOT DRIP[ NOT TOO THIN.] NOW HERE`S THE TRICK. WHEN THAT FIRST
COAT IS TOUCH DRY, BUT NOT CURED, APPLY A SECOND COAT OVER IT. IT WILL
BIND TO THE FIRST COAT AND GIVE A LOVELY SMOOTH FINISH. DO NOT
SANDPAPER THE FIRST GLOSS/SEMIGLOSS COAT. I DID NOT BELIEVE YOU COULD
PAINT GLOSS ON GLOSS BUT IT DOES WORK..AS LONG AS YOU DONT LEAVE IT
TOO LONG. HAPPY PAINTING