How do I flatten resin-coated photographs?
Q. Dear Donia,
A. Dear Dean,
There is only one way to flatten a resin-coated (those newer photographs that are like plastic) print and that is with heat.
DO NOT humidify the photo. Rather, use gentle heat (not above 180 degrees). You can do this with an iron between blank newsprint or unprinted photocopy/printer paper. Be sure the iron is not set for steam. With the iron on, apply gentle pressure for about 30-45 seconds making sure the iron is moving so you don't scorch anything. I would start at a lower heat and slowly raise it until you find the sweet spot. Once done ironing, put the photo immediately between blotter and under weights to cool flat.
As with humidifying and flattening, go slowly until you know what you are doing and always be careful.
I hope this helps.