Great work on the B24 Wojtek, it amazes me what you do with card models !
Rob, a rough texture is nothing to do with it being a couble-action 'brush - it should have more control than a single action. the texture will be cause by either the pressure, or the distance you spray from. There's no straight answer as to how to cure this, as it depends on the paint, how it's thinned etc. Just practice on something to get the best pressure and distance for the desired finish with the paint mix being used. Start by reducing the pressure if it's above , say, 15 psi. If it's a fixed pressure, alternate the diustance you spray from, and see what result you get.
You really need to practice to get used to the variations possible with the position of the trigger. Remember, pressing down only lets the air through, pulling back regulates the amount of paint flow. The further back the trigger, the more the paint will flow.