[Zeke]

Use a smaller tip than what you would use for paint. They even make a "fine finish" tip that has a double orifice so the stain is atomized a bit and distributed more evenly. That tip will cost double what a regular tip does. I'd recommend a .015 at the most for either type. Strain your material through a strainer bag. No kidding because there is a lot of junk in paints and stains that will clog a small tip.

You don't have to have a lot of overspray. Hold the gun at 12" and never more than 80 degrees perpendicular to the fence. IOW, don't "fan" your strokes. Orient the tip sideways and go up and down with the boards, never across. After getting used to the right amount of material applied, it should go well without too much back brushing. Spraying stain on a thirsty surface will always come out lighter than if you brushed it. Keep a brush handy for any runs. Maybe do 2 coats while its wet. Normally you would wait until your material is dry for the 2nd coat, but with stain you want to saturate, not build.