Will I need to thin down Cabot semi solid stain to cover deck railings and pergola?

Possibly. We expect that 5-10% thinning will make enough difference for your project. There are a few things you can do to paint/stain to ensure it sprays well. 1 - Stir the paint/stain with a mixer in a drill until its creamy smooth, about 1 min per gallon 2 - Strain the paint/stain before it goes into the sprayer 3 - Add a flow agent, like Floetrol or Paint Easy (this is not a thinner) for latex paint – Penetrol for oil based 4 - Or you can thin, even as little as 5% can make a big difference, water for water based materials and mineral spirit for oil based material.