You can’t keep wasps from building a nest. The surface would have to be slick as owl snot.
The idea is to make them less likely to do so, even if I can't prevent it 100%. Kind of like how businesses pump out certain music or high-pitched sounds so teenagers don't hang around. This is why I thought changing the surface on the underside might make them less inclined to try a nest there. Or perhaps closing off the underside area with some plywood? My guess is the wasps liked it because the underside is protected by a 2"-3" piece of wood on four sides.You can’t keep wasps from building a nest. The surface would have to be slick as owl snot.