The plasti spray hardens the foam so that it's no longer soft and plush. Once you have plucked the foam to your desired dimensions, you can then spray it in order to set it so that it no longer separates. Otherwise, over time, the pluck foam will eventually pull apart. There are several threads here that are dedicated to spraying the foam.
The only parts that are sprayed are the bottom pieces. The top part should remain soft and foamy so it can easily compress against whatever you put inside.
Not the best pictures since the only thing that is left is the outside perimeter and the bottom, but it does show how the phone that has been sprayed around the perimeter has darkened compared to the foam on the top.
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