Re: What time of day to apply Oxalic Acid
Early morning seems to work best, you can get all the bees this way. With a small apiary arrive at the yard as you begin getting the first light of the day, and before starting close off all the entrances with cloth or paper towels to trap all of the bees inside the boxes (this would be with SBB's so they have ventilation while waiting). As the sun comes up you have plenty of daylight available as you work through the hives.
Late evening is a bit tougher. You will still have returning bees right up until dark and you could also have bearding to deal with. This time of day could also leave you fumbling around in the dark with some of the hives. Oxalic Acid Vapor is nasty stuff, I think it's important to have light and see clearly what you're doing.
To everything there is a season....