Paul
Polypropylene is hydrophobic so repels any water based treatment along with water based soiling. It is also oleophilic, ie it loves oil so attracts it to become easily stained.
At present, polypropylene may just as well be treated with plain water as the protector will not be accepted. But there has been talk around for a few years about chemically etched fibres that may accept flouro chemical protectors. Haven't come across any yet though. It has also been suggested that the Dry Fusion system using Activator or Bactoshield will heat bond the protector to the fibre to allow an effective soil resist treatment on polyprop. Although I use this system, I have not carried out any research myself to suggest that this works well. It certainly works for me on wool and nylon though.
You would need to ask the manufacturers about new generation nano based protectors on polyprop.
Safe and happy protecting

The Ken