I have several kits that include Walthers one piece smokestack which is also sold by itself. I had never been able to create a good mortaring effect. Since mortar washes by neccesity have to be thinned, gravity would cause the mortar mix to settle toward the bottom of the smokestack if it was applied with the smokestack vertical. If laid horizontally, it would settle on the bottom of the stack leaving a white streak on one side and virtually nothing on the top side.
This morning I found a solution. After appying the mortar, I placed it on my Showtime rotisierre. It just barely fit on the spit. I wedged some Kleenex in the larger end to prevent it from swiveling as it rotated. I turned it on, with it set on NO HEAT of course, and let it go. "Set it and forget it". I came back a half hour later and it had dried evenly around the smokestack. It was a little faint so I applied a second coat and and repeated the process. It came out just fine. Any device could be used that would rotate the smokestack while it is in a horizontal position.