Many years ago I used roofing cement (I'm sure it was identical to Black Jack, just a different brand) to seal an old VW bug from the outside and inside. It did eventually dry. I don't remember how long the stink lasted, but it eventually went away. But I did not sandwich it between metal and wood. I think I put paper or cardboard or something like that on top and then carpet. The problem with a coating like that is that if any water/moisture sneaks in it will be trapped and rust out the metal in that area. A bus has a lot of surface area to be coated, which means a lot of tar. That may stink quite a bit. I personally would not go that route. Once you put that stuff on there is no turning back!