I have a super high-tech way to flatten out my cyanotypes after they get developed. LOL. Usually, the paper warps because they get developed by dunking them in a water bath. After 48 hrs in an old book, they should be ready for the next stage.
What I do is after I’ve washed the image in a tub or under a sink for a few minutes, I hang the print to dry in a dark place for just a few minutes, until the print is mostly dry, but there is still some moisture in it. Then, I put the print in a heavy book and press it with some weight for 24 to 48 hours. That way, the drying completes while it is pressed completely flat.
Also, for another tip, it’s best to use a thick paper stock, I used 250 gsm for these prints, but will be trying 300gsm next time I need to restock my paper.
Anyway, I hope this was helpful. Cheers.
– Roberto Scozzari