I am going to make a dummy cake, I know royal icing is the traditional medium, but has anyone tried to use gum paste, does it last?

I've used fondant to cover one, but not gumpaste. I've also used lightweight plaster.

I think gumpaste would dry too brittle. I have added a bit of tylose to the fondant though to firm it up before applying it to the foam.

Any time I'm covering a dummy cake with fondant, I thin some piping gel with a little warm water, lightly brush it on the foam, especially around the edges, and apply fondant in the same way I would a real cake. Gumpaste would not work for covering a cake. The fondant will eventually dry and last quite a while.

Can you imagine trying to smooth and gently tug the gumpaste as it's drying? No way, I wouldn't try it.

fondant is perfectly fine to use for this. I've done it, and I just lightly spritzed the dummy with water, and applied fondant as normal.
As long as the dummy is kept out of direct sunlight and in a cool, dry place it can theoretically last for years. Although colours will eventually fade with time.
I used a dummy tier for the top tier of my dad's golf birthday cake, (in my pics) and it was made on September 28th, and I just recently stripped the fondant off it to re-use. It looked exactly the same as the day it was made.

Royal icing is more prone to cracking when dried, especially if it's moved and bumped.

Mama D

Thanks so much for all your ideas. I think I will try the fondant, this particular dummy cake is just for display in my kitchen of a really pretty cake plate.

I am also attempting to make a dummy cake. Is it better to cover the styrofoam dummy in plastic wrap before covereing it in fondant or will the dummy be reusbale even if you put the fondant right on the dummy. Thanks!

I don't cover mine with plastic wrap. I usually use just a bit of water to hold the fondant on. If you are going to be changing the cake design often rub shortening on the foam instead of using water. It usually peels off easily.
Make sure you soften the upper edge first. It prevents the fondant from tearing when you are applying it. I just put my cakes in the dishwasher to clean them off. It's quick, easy and the fondant just melts away.

I just spritz the dummy with water prior to adhere the fondant. I've got dummies that I've been reusing for 10 years.