How are you making your prepreg? If your resin viscosity is too low at room temperature (which is probably the case if you wet impregnated the fibers at room temp.), then a release paper will not solve your problem. You will need to freeze the resin to get the liner to release cleanly. You can do this in a freezer, a vortex tube cooler, or with a freeze spray.

We are making it exactly as you described. We lay a piece of mylar down on a smooth glass plate. Then we put 0.040" shims on each side to set the thickness. We dab the low viscosity epoxy on the mylar and then stick the carbon fiber material on top of the resin. We then add more resin on top, put another sheet of mylar over everything, and use a steel roller to smooth it out to the shim thickness. We then let it air cure for a few days.

So you're saying even with release paper this wouldnt work? Do you mean to freeze the resin after it's been cured to release the mylar?

Thanks,
M

Okay, so you are not really making a prepreg, but making a wet-laid panel that is cured at room temp., and you have a release problem after cure. Release paper will not help you. It is porous to air and sensitive to humidity changes. I'm surprised that the Mylar cannot be peeled-off. That means that the Mylar was probably surface treated to make it bondable or to allow writing on it. There are release coated mylars available. You can also use mold release on the mylar. If you do not need the transparency (I suspect that you do to see where to sweep bubbles out), then an excellent, and reusable, release film is teflon impregnated fiberglass.

You can get polyethylene film on ebay for about 100 times cheaper than the overpriced stuff that's dedicated for vacuum bagging. I think most plastics including polyethylene naturally don't stick to epoxy. I even used food-grade plastic wrap from my kitchen before and it worked fine - only problem was that it was too thin, clingy, tended to wrinkle, and 12" just wasn't wide enough for most applications.

Beware of Cheap Polyethelene film, the stuff varies in thickness across the sheet, and can be porous.
If you are looking for a quality finish then a release coated mylar,is worth the extra money
If you do not care that much about the finish, then Bagging film from your friendly fiberglass supply will do what you need.
B.E.