I made a batch of Nic Lodge's gumpaste, and tried to cover a cake pan with it (to make a "lid" for my box cake) and it was very firm. I had the toughest time rolling it out. Once I finally got it rolled out, it barely covered the pan, and all the corners cracked.
Does anybody know how to make your gumpaste a little softer? And how to roll it out and cover a square cake pan without the corners cracking?
Gumpaste is not meant to be used to cover cakes. It's for use in small amounts when making flowers, leaves, or other decorations that will be attached to the cake.
Fondant is what you need to cover cakes. It has less "gum" ingredients than gumpaste, so it is much softer and easier to roll. There are a lot of commercial brands available or you can make your own. Which one you use will be personal taste and will take some experimentation.
From your post - I'm thinking you are making a hard lid cover - not covering a cake....
If your gumpaste is too tough - you can add shortening to it when you kneed it to make it a bit softer. This should also stop the cracking issue.
Hope this helps....
I like to use a 50/50 mix of gumpaste & fondant. It gives you a nice, hard, strong lid, and you have a little more time to work with it before it starts to dry.
When I do use straight gumpaste, I often roll it out and smear a thin layer of Crisco over the surface of it. It slows down the drying and gives me longer to work with it before it dries too much. Once it starts to dry, handle it as little as possible, as that seems to be what causes the most cracking for me.
You can add tylose to fondant. That is how Sharon makes her box top lids. They turn out great.