I try all sorts of experimentation, but I can never get it to paint smoothly. Even with a lot of thinning, it still is splattery compared to what comes out of a spray can. What am I doing wrong?

What airbrush? What pressure are you using? What kind of paint?

Paasche double-action with Model Master Acryl paint.

Tom, have you fully stripped your brush and cleaned it well? If you have other needles change it out and see if that helps too. Also check your pressure and make sure it's not too low, I find that 20 psi or so works well. I

you might want to try some bees wax on the threads of the (outer nozzel) not the one that holds the needle this will seal the airbrush and not allow a siphon effect this helps with my iwata revolution

What are you thinning your paint with and at what ratio?

With a magnifying glass ( preferrably 10x ) check your nozzle end for any chipping, burrs or hairline cracks and also check your needle for any burrs or a slightly bent tip.
These parts can become damaged very easily and go unoticed by the naked eye.

I've been air brushing with a Paasche 1H air gun for more then 40 years. After every session you should clean your needle, cone, and either cup or jar thoroughly. Running Alcohol or lacquer thinner through it is not cleaning them correctly. More often then not some dry paint particles will be left over to cause havoc your next paint session.

I use a #3 tip, needle and cone. the #1 system is for water colors. They also have a #5 which is for large area coverage, and not really needed for modeling.

For Model Master acrylic paints, you should thin them out 3 parts paint to 2 parts thinner. I thin Tamiya acrylics out 1:1. I just find the MM acrylics to be thinner in the bottle, but that's me. I prefer Tamiya's X20-A or even their lacquer thinner works great. Using 70% Denature Alcohol, I find that it evaporates to quickly for a smooth application. You need to add some retarder to the bottle that you can get a artist supply store. I'm not a big fan of distilled water for thinning either Tamiya or Model Master acrylic paints.

I'm assuming that you're using a air compressor with a regulator. If it has a storage tank, so much the better as moisture even with a trap is much less a concern as the water settles to the bottom of the tank. A tankless compressor runs continuously,and gets hot, causing even more water condensation. All the water is pushed out of the compressor, so make sure that you have a good moisture trap and empty it during the painting session as well.

As previously mentioned, make sure that your needle and cone are not damaged. You can checkout your needle just by rolling the tip over a perfectly flat plate. The cone needs to be examined with a magnifier of some sorts.

I air brush @ 18 psi. You should never need to go past 20 psi, or you have a problem of blockage or your paint isn't thinned out correctly.

The last issue to be concerned with is the paint itself. you need to make sure that it's completely mixed with no particles floating around. Do not pour the paint from the bottle into your cup. Use a syringe or eye dropper. That way you can properly measure it, and there is chance for any bits and pieces to get into your cup.

Now to actually paint. Close the cone down all the way on the needle carefully. Now push down on the air button. you should get a full stream of steady air. Now open the cone up a few turns and start to see if paint comes out of the cone. Keep on opening it up until you get a nice consistent amount of paint for the application you want. You should hold the gun no more then 3 inches from the surface to be painted.

This should give you a consistent and smooth flow of paint. Hope I was able to help.
Joe