A reader emailed me to ask why I was going to the trouble of brushing this paint. Why not simply spray? I'd be done in a few minutes!
Well, there's actually a good reason. On rough, unfinished surfaces, properly brushed paint will adhere far better than sprayed paint. By “properly” I mean brushed from multiple directions with the brush held nearly perpendicular to the surface, and a light pressure applied. When brushed in this way (assuming a decent, fine-bristled brush) the brush's bristles will force paint into every nook and cranny on that rough surface, wetting it all. When the paint dries, the brushed paint has a large number of nearly microscopic wedges and braces against the texture in the underlying surface. Sprayed paint simply doesn't do this. Instead, it forms a thin film just above the pits in the rough surface, without ever wetting the inside of those pits. Sprayed paint's adhesion is entirely from the chemical adhesion to those surfaces it did wet. Properly brushed paint has (a) a larger wetted surface area, and (b) all those mechanical interlocks with the rough surface. The properly brushed paint is much more resistant to abrasion, more waterproof, and will last longer.
The “self-leveling” (making a smooth surface) of modern paints is extraordinarily good. With a quality modern paint, brush marks are almost never a problem. This certainly wasn't true when I was a kid; getting the brush marks out of brushed paint was a bit of an art back then. Paint chemistry has come a long way!
It's worth noting that the argument for brushing over spraying really only applies for rough, textured surfaces – and even then, only if durability is the objective. Anytime you're painting with appearance as the main objective, spraying is going to be better. That's especially true if you've got access to a high volume, low pressure (HVLP) sprayer – they make it quite easy to produce paint finishes that look absolutely perfect.
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