Fondant glue is an interesting topic I must say. Fondant is so widely used as cake decoration, not only as a cake covering and decoration on fondant covered cakes, but also as decoration on buttercream covered cakes.
Here is an example of how I used fondant decoration on buttercream covered cake. When buttercream is the base for sticking fondant decoration or cut outs, I do not use any 'glue'. Small flat fondant cut outs stick on buttercream icing without the help of any special medium such as fondant glue.
See the circle blue, white and green motifs on the cake? Those are cut out from fondant and the entire cake is covered in buttercream. In fact, the 'happy birthday' plaque is also cut out from fondant, and I did not need any 'glue' to stick it onto the buttercream covered cake.
The tips I am sharing below are specifically for sticking fondant cut outs or decoration on fondant covered cakes. I did not learn all this these at one time, I discovered them along the way, over the last decade or so of my cake decorating journey.
There are 3 types of 'fondant glue' I use in my cake decorating projects.
So when do I use each of the above types of glue?
Frankly speaking, since I discovered the CMC glue and shortening, I hardly use water anymore. I use the CMC glue for heavier decorations like fondant bows and gumpaste flowers. CMC glue dries hard and faster than water, so its very effective for heavier decorations.
With shortening, there is a risk of the decorations not holding in place and sliding off. Therefore, I tend to limit the usage for fondant cuts and lettering as it gives me the much needed flexibility of being able to remove and adjust them to perfection!
Here is a cake covered in fondant I made not too long ago. This is a remake from an image provided by my customer. Credit goes to the original designer.
Notice the large bow and the tiny pink bows on Minnie's face around the cake? Those are attached with CMC glue. They are heavier than simple cut outs, so CMC helps to 'glue' them in place. The birthday girl's name, MEENA, is attached to the cake with shortening. These letter cut outs are light, and the tendency of having to adjust the letters so that they are in a straight line is higher, so shortening makes it easier to remove and reattach without leaving any marks on the cake.
There you go, that's my sharing on how to stick fondant decoration on cakes. Hope this helps in your fondant decorating projects!