Yes, there will be slight changes in film density. However, the film density through the weld should be less than that of the adjacent pipe since the weld reinforcement will increase the thickness along the centerline of the weld face. Slag trapped between the backing ring and the ID of the pipe will increase the "thickness" very slightly since the slag is more dense than the air that would otherwise fill the void. Since the slag is slightly more dense than air (but not as dense as metal), it will absorb more radiation so less radiation will strike the film thereby producing a slight decrease in film density.
Al