You have to apply spray 15-20 mins before the act, u can do it before selecting gal...just tell u have to pee.
Applying spray wont come in way for you to achieve erection, as it depends on blood flow in penis veins which our brain directs when we are aroused.
From what I read last, The Spray has to be applied once you get an erection as it has to be applied on nerves near your shaft. If you already apply it before you get an erection, And the nerves near your shaft become already Numb one may have problems getting an erection. Albeit someone who has used it personally may be able to say it with more confidence. I haven't used it.