If the steam is coming out in a strong, steady stream, stick a spoon in the spout to diffuse it. That way it won’t be as likely to warp the paper with a thick stream of moist heat. If you don’t have a tea kettle, simply boil a small pot of water until it produces steam.
If your envelope is a long business-sized envelope, run the entire thing over the stream of steam to make sure each portion of the glue gets loosened. Don’t hold the envelope in the steam for more than 20 seconds, or the paper will begin to warp.
If the flap doesn’t seem to be giving, and it tears slightly instead of easily lifting, place it back under the steam so you don’t end up ripping it.
You could also iron the envelope flat to prevent it from wrinkling. If you choose to do this, be sure not to leave the iron on the paper for more than a few seconds, since the concentrated heat can cause the paper to turn yellow or burn if you’re not careful.
Use a gluestick. Since the glue in a gluestick is relatively dry, you can use it to reseal the envelope quite discreetly. Apply glue to the edge of the flap and close the envelope. Good as new. Use a wet glue. White school glue, superglue, or another type of wet glue is fine to use if you don’t have a gluestick. Just make sure you use as little as possible, so the envelope doesn’t get wrinkled with moisture.
In order for this method to work, you must use the freezer, not the refrigerator. The refrigerator’s temperature isn’t cold enough to make the glue unstick. If you don’t have a freezer available, try placing the envelope in a plastic bag, then submerging it in a bowl full of ice water. This is quite risky, since a leak in the bag could cause water to flow inside and ruin the envelope and its contents.
If the flap won’t stick when you try to reseal it, use a gluestick to keep it shut. If you don’t have a gluestick, use a very light application of white school glue or superglue to close it.
