Hi Petra,
It sounds like you are trying to create resin geodes?  
Because resin wants to ‘move’ when its wet, you have to find a way to contain it until it starts to cure.  If I were to try this, I would put down stripes of hot glue to act as a corral for the resin.  Once it has cured, you can apply a second layer to cover up the first layer and the glue.
I have seen other artists us clear plexiglass to help with this as well.
Is this what you were looking for?