Here's the capx file to demonstrate what I'm trying to do.http://www.wethesqueemish.com/construct/masking/MaskingIssue.capx
So to break it down a little clearer:
I've made a dirt layer.
I want to mask that dirt layer to another layer that has various platforms (made from a white tiled object).
I've used the method from the above tutorial, putting the white platforms on Destination Out and having the layer force texture and non transparent.
This works as far as the dirt texture goes, it's masking to the shape of the white platforms perfectly.
The problem is though, the background is no longer transparent, so I can't see my sky layer which is below the others.
Hopefully that's clearer, thanks for your help.