New answers tagged cs6
With Live Paint Selection Tool you can select not only segments of a fill but segments of a stoke as well - just click on a segment of a stroke that you want to get rid of, and then either select a 'None' color, or set stroke width to 0 on the Stroke panel.
Make a group, give it low opacity and continue by creating your shape layers inside that group. I'd use the same method in older PS versions as well. Here's a picture where all the shape layers have 100% opacity and they are inside a group with 38% opacity that affects all the layers in it.
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