I've been painting for decades now, full time for a couple of summers long ago, but this is the first time I've painted a shingled building and it's giving me conniptions. This is a barn, where it hasn't been reshingled recently the cedar shingles are heavily eroded and have lots of texture.

I'm using a medium stiff 4" poly brush (Wooster) and having to do a lot of brushing in fill in all the eroded pits and raised grain, slow going, but ultimately successful. Paint just laid on the surface, as with a sprayer or brushing over the surface doesn't fill the pits, I often have to jab with the end of the brush to get complete coverage. The shingles end up taking a lot of paint. Heavier application makes it easier, but exacerbates the problem I'm having with drips. With all of this brushing I'm also getting a lot of paint in the gaps between the shingles, which then proceeds to drip onto the course below over the next 5-20 minutes. I'm dealing with this by going back and picking up the drips, but it is a pita, especially on a long ladder.

Is there a different technique to use to avoid this dripping? The problem is mostly with shingles that are far enough apart to hold paint, but too close to get the brush bristles into the cracks to clear them out. There is no problem with drips or sags on ungapped surfaces so I think the paint is OK (also have the same behavior with oil-based primer and latex top coat). On the one hand it's only the barn and probably no one will notice, but on the other it looks like amateur hour out there and it's frustrating.