Since POP is primarily a UV sensitive process you can get a lot more effective printing contrast with most films by developing in a staining developer. I would recommend a 5:3:100 dilution of Pyrocat-HD.
Originally Posted by Roger Hicks
If the scene you are photographing is one of normal contrast you can get plenty of contrast for POP with most ISO 400 films, including HP5+, TRI-X 320 and JancC 400. However, if you are photograhing a low contrast scene the only ISO 400 film (that I know) that will give you enough contrast for POP is TMAX 400.
If you can live with its speed Ilford FP4+ is an excellent film for developing to a high CI.