Polaroid makes many B&W films with speeds from ISO 50 to ISO 3000. The Polaroid material that producs a negative as well as a print is Polaroid type 55. For the best negative exposure rate it (for metering purposes) at ISO 32 or 25. If you are going to do very long exposures (like over 4 to 5 seconds) you get into severe reciprocity failure problems.

Yes the negative needs to be cleared ofthe processing solution in either a sodium sulfite solution or running water.