I have seen real old unfinished EPP projects and some of the different papers used was amazing. Mostly newspaper was used and old sewing pattern paper which is much heavier the todays, magazine paper, the list goes on. People used whatever was at hand. WE are very lucky today to have so many options. Try them all to find what you like to use. Everyone seems to be in aggreement that they like to do this carry along piecing project. have fun.
peace :D