Hi,
I was just making a short animation of a space Battle and was compositiong the starship engines and found that when I joined the two render layers together with a MIX node at factor 0.5 the engines would show through the ship. I have tried Overlay and screen nodes but the results were much the same.
I am also using the blender internal renderer in blender 2.65,
Tom
Use “Alpha Over.” The image of the engines needs to be masked by the ship. Also see “Z-depth.”
Thanks for the reply, do I use nodes to do this?
Tom
Ive tried to use the alpha over and it works from behing but from some angles I get the same result as before like in the picture(Below)
Tom
You forgot the Alpha! 
Plug the alpha channel into the Factor input socket and you should be good to go. (This node-type is also sometimes used to achieve simple, uniform translucency … hence, it can be an input mask or a single number.)
“Z-depth” can also be used to distinguish which of two channels is “in front of” the others. Take a careful look at the various compositing nodes. Essential Blender Compositing is also a very good book to read.
