Here’s a trick for avoiding runs when brushing a finish onto a vertical surface. Brush back over the areas where the finish is so thick that it runs to pick up some of the excess with the brush’s bristles. Then wipe off this excess onto a clean cloth you hold in your other hand, as shown in the accompanying picture.
You may need to do this several times to remove enough finish so it doesn’t run anymore.
Always remember that you can’t see what is happening on the brushed surface unless you look at it in a reflected light. You may need to arrange some lights, and you will probably have to move your head around to see a good reflection.