dead easy mate, new layer, use a standard round brush flattened in the brush tip shape option, play with scattering (and give it a little angle jitter to add a little random direction) paint your rain and reduce the opacity to around 25-20%. Depending on the painting you might also want to set this layer to screen or overlay for a really subtle effect. You can also add multiple layers of different sized/opacity strokes to add depth to the rain. Hope this helps
I used to do it exactly that way, in previous speed paints, but it never turned out looking as good as yours. I think my rain drops were too big and there were too few of them. Maybe I should try it again now since I have improved some at speed painting. Thanks, I really appreciate your feedback