When frixion pens first came on the market, we tested them, and found ironing did erase the markings, and yes the freezer brought the marks back, but once the item has been washed the marks are permanently gone. You can also use your blow dryer to remove the marks, should they return from the cold. If you are flying with your items the marks could come back from the cold if packed in your luggage - again the blow dryer works. Becareful if you are sending your items to a quilt show and you are not attending - the marks could be there for all to see if your items are exposed to the cold. This actually happened to our quilt submission and when it came back our administrator was horrified, that it had actually been hung up with the marks showing.......OMGoodness!
I found some of the pens didn't last very long, I believe it has something to do with shelf life, therefore I don't take the first package off the shelf, you may want to take the last one in the pile or off the hook - also keep your pens in a zip lock bag, away from the air - we tried them standing up in a pen holder and then some in zip lock bags, the zip lock pens are still working and we have had to throw out most of the one from the pen container....just saying