Mud dauber's make their nests out of mud, so a high pressure sprayer should wash them away. My parents have a wooden house and use a high pressure sprayer about 1x/year to wash all the bugs and stuff off, works great. It will take old chipped paint off, most anything will do that too.

You know mud daubers aren't a threat, You could just leave them, they are physically able to sting, but very rarely do so. Unless you pick them up in your hand they shouldn't sting you. They are considered beneficial insects, they also eat spiders which you may not want around like the black widow.

I am in California. While mud daubers may be beneficial because they catch spiders and insects, they do harm to electrical tools, machine parts, non-operational vehicles, and the like. We have a dam on our property so I can't really do anything about the mud, but I would like to get rid of the daubers. According to people here, I can't do that. Why not?

seajaih -
You can't get rid of them because they are EVERYWHERE! For every nest you see under a junker car's fender there are a dozen on rocks and in trees.

Caulking and screening the places where you store tools and working vehicles helps prevent the nests.