Krylon Crystal Clear Acrylic Spray can be used to harden plasticine.
Leave your model for 10 minutes after spraying and then decide if you think it needs a second coat (or more). When you have finished coating your model it needs to be left to try for two hours.
Never try to bake plasticine in order to harden it, it is oil based and will not harden this way but you will be left with toxic fumes!
Polymer clay is an alternative for a model that you want to keep. It is easy to work with and can be baked, as it comes in many different colours and it offers greater scope. It is not meant for items made for food or drink of course.