iv never u used marangue on my cake but i know if you use normal buttercream on your cakes then you can pipe royal icing on to it not strait away just leave the buttercream to set for a bit. what i do is mix a bit of the royal icing powder in to my buttercream just to get a bit of a firmer finish on the cake this allows the royal icing piping on a better surface to sit on. hope that was helpful.
