Thanks a lot aleon for this info!

It really helped a lot. The hint about the buttons really made calm me down and explains a lot.
Unfortunately I do not understand any spanish word on the site you referenced but the pics also helped a lot.
The technique shown on the pics is quite similiar as the one I described in
my tutorial about casting, but differs in one single but for me very important thing: using a vaccum machine. Just when reading your post I was thinking on some research I made some time ago about a suitable vaccum machine for doing better castings. I actually found some but the drawback was the price: several thousands (!) Euros. So I droped this idea.
When I then saw the pics in the spanish forum and spotted the vaccum machine a glance arise in my eyes: such a vaccum machine we actually do have in our kitchen!
Never spend a thought about using it for this purpose.

Many thanks for giving me this hint.

With my next casts I will for sure make some experiments with it hoping for some better results.