The way Poser saves the prop is terrible with all the embedded geometry. However, I noticed that after you remove embedded geometry, Poser will save it without the embedded information. What I would do is save the prop out again with the materials you want in the preview material zone cause you can't delete it as far as I can tell. Not sure it will work like you want, but it's worth a try. I personally remove the geometry from the file first and then tweak my materials. Hope it helps.

Have a creative day!

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My Lil' Store