Hello Grancru, What I've learned from my experience making molds is, yes you can make it too thick. It then follows that you can make it too thin also. I look for about a pancake batter consistancy to pour well.

One tip is to let the plaster and water sit together WITHOUT stirring for a few minutes. This is called slaking. Then only stir enough to get all the POP wet. Too much stirring makes the plaster set faster too. So go light on your stirring.

Here is a link that might be useful: Great Plaster site for tips

When you mix up your plaster of paris, add about a tablespoon of vinegar to about a quart of water. This will give you more time to work with the plaster before it hardens.