It seems that applying the Chemzone Sludge Skin will change the base appearance of both the battle maul and power maul to that of the other. Have not tested other skins but the visual change of the model to other maul can be visually confusing.
This also causes the animation of the power maul with the skin to look horrible as the ogryn is no longer pushing the button to turn on the power maul but instead shoving his thumb into the handle of the maul.
