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Good question. The answer can vary by resin system. The manufacturer should supply a data sheet with a specified “out-life”. This can be very short (days) for some systems such as tooling prepregs but can also be a number of months for high temperature systems. If you know that the material will be consumed within the outlife it is perfectly acceptable to store wrapped up at room temperature as long as your location doesn’t get too hot.
Most of the time, however, you will need to store prepreg in the freezer. The prepreg should be contained in a moisture impermeable wrapping (preferably also airtight) prior to freezing. When defrosting for use the material should be left in its packaging until you are sure that the temperature is well above the dew-point in the room, this might be a couple of hours for some flat sheets up to a day or more for the largest rolls. If you take the prepreg out of the wrapping before it has defrosted water will condensate on the cool surfaces which may cause problems with the cure. For this reason you should also inspect the packaging for any damage and repair immediately. It is good practice to keep records with each roll where you monitor its so-called “out-life”, each day out of the freezer should be logged.