This was my first brisket on a WSM,and there really was no mustard needed. I am not usually one to brag. But I think I nailed it the first time. I pretty much followed the directions on this websight. Smoke to about 150-160,and wrap with foil,and cook in the oven to 200-210.
I did put mustard on the point yesterday,(after it sat wrapped in foil in the fridge overnight)going for burnt ends. But I don't think you are supposed to do it with meat that has already been smoked. I peaked after an hour or so,and most of the mustard was still there. So I pulled it off,and dabbed up the mustard with paper towels,and put it back on the smoker,and let it cook.
I think a big part of why it came out so good is i made a mixture of beef broth/bullion,and Worcestershire sauce,and kept injecting it every hour or so. I also mixed some of that with apple juice,and olive oil and kept it sprayed down with a spray bottle.
My next one I am definitely gonna try mustard. But I'm only gonna slather it on raw meat,and i will go easy on it..