For days now a procession of wasps have been landing on the top railing of our cedar deck and poking around for three or four minutes at time. Finally, this afternoon I figured out what they are up to.
If you look very carefully at the photo above you may be able to see that the area in front of the wasp is a lighter color. The wood is not painted. That grayish-white color is actually the color of fresh cedar that is just beginning to weather. What's happening is the wasps are chewing off this weathered, oxidized layer. In the process they leave behind areas of raw wood that shows up in this photo as a blond color.
They are doing this all over the surface of the railing, but they tend to focus most on the curved edge, probably because they can get their mandibles around that contour more easily. As they move along they roll up a little gray ball out of the stuff and then fly off. I'll keep trying to get a shot of that. Presumably they are using this material to manufacture nests. I've never seen this behavior before, so if someone has a different take on it, please post it here as a comment.