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Making a wedding cake the bride would like a waterfall stencil on all tiers once cakes are iced when can I go ahead and stencil with royal icing any tips on this would be appreciated , I believe there is a whitener you can add to royal icing is this correct and what kind of consistency should it be , how many days before wedding can I do this
How soon you can do this depends on the type of cakes you are using. Not all cakes have the same shelf life so it's best to use something sturdy like mud cake or maderia which you can work on for a good five days before delivering. Sponge cakes have a shelf life of five days from baking to eating, mud cakes depending on recipe will have a good seven days and madeira, again depending on recipe, anywhere between seven to ten days.
Personally, I would keep the fillings simple and not fill excessively.
If you're using fondant to cover the cakes with, let it dry to form a crust for a day to give a good platform for the royal icing. For royal cing techniques and consistencies please take a peek at the excellent tutorials here:
Search in the techniques section of Cakeflix.
You shouldn't really need a whitener for royal icing. This is something which would normally be used for buttercream to obliterate the yellow tinge created by the butter. The product is a food whitener:
Google the product by name to see other suppliers.