I work primarily outdoors, and carry pepperspray as part of my work kit. I have had good results with pepperspray that is ejected in a stream or spray. Foggers, for me, have been useless, because of their susceptibility to breezes. Make sure you hit the eyes, nose, and mouth.

I used pepperspray as a first defense on all dogs, until the day I was caught offguard and offduty, and was chewed up. Since that attack, the pepperspray only comes out for dogs that I think I can punt. Large dogs have gotten lead when their owners were not there to control them.
If you wait until the dog bites, you will not get a clean shot.