On my first try of making silicone moulds, I got some problems with the hardener and parts of the mould remained sticky. This time, I tried to mix the hardener to the silicone more thoroughly, but still got some sticky parts. However, I found a fix for the problem. It occurred to me that as the hardener mixed in the silicone hardens it, hardener applied with a brush to the sticky surface should do the same. I tried it and it worked.
The uneven parts in the picture were the ones that remained sticky. They have a slightly different color, because the hardener is blue (to make it easier to see if it has been mixed in the silicone well enough).
Update on the book:
I have written most of the chapters about mould making and casting, plus quite a lot about materials. I currently have about 9,000 words written, plus 70 photos. There will be quite a lot of photos in the book, because it is so much easier to show things in pictures. It should also be easier for people with less than perfect command of English. The current plan is to have the book available only in English, although a Finnish version is also possible later, if I feel like translating the book and have time to do it.
The main change to the original plan is that I won't include any example projects. The book will be long enough with the other material. I'm thinking about a possibility of making another book later about example projects. First I need to get this one finished and see how much interest there is.