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Polypropylene is polypropylene. ...
Not really. It can be hard and shiny like in ABS sailboard skins and Australian bank notes or soft and fluffy like polar fleece.
I'm guessing a polypro rash guard should be a but denser and a bit shinier than thermal undies.
I would expect polypro thermal undies to work quite well as an under wetsuit layer, similar to the Fireskin on my RipCurl wetsuit. It won't hurt to give the undies a try when the weather gets a bit colder.