The cans you buy just spray down. I'd need an insecticide that can somehow get through the slots and spay the underside.
Don't think of the deck as an impenetrable barrier. If the boards are just screwed down, unscrew them. If nailed down, pry them up carefully with the right tool.
Hmm very good point. I guess if I do it at night the wasps won't try to sting me?
Do it when you can see the fuckers and run if you need to.
When I was 12 I once took out a hornet nest with a water hose by simply doing rounds of blasting them with water then running away while they tried to chase me.