You're right in thinking the spray won't reach the nest. It won't. Gravity and tight wood joints will make it all but impossible for a liquid spray to get where it needs to be. But in our wasp control article, you'll see a product called Drione that's perfect for the task at hand. If you view the video on how this product "floats" around after applied, you'll understand why it will do the job. Apply it with one of the Hand Dusters into all the cracks and crevices you find and you'll get any wasps in the area real good. The Drione will seek all the voids and spaces where they nest. In fact, if you do this treatment once in the spring and then again once in the fall, you should be able to keep them under control and the house wasps nest free.