Flatten curved postcardd

Hello Postcrossers,
Dad gave me some unwritten old-ish postcards (likely from the 60s and 70s), some are really nice but I cannot send them because they are curved.
I read somewhere online that one should dampen the concave side by keeping it over boiling water, then press the postcards under a weight for a few hours. I tried but the result was only temporary and after the long pressing step the postcards were as curved as in the beginning.
Do you think they can be flattened again? Any tips?
I searched the forum and did not find information about this topic, however if this has been discussed already, please redirect me to the relevant thread.

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Damp watercolor paper buckles but if left taped down it flattens. I would put a damp cloth over and iron it then quickly (while warm) put it on a cold, flat surface with books on top. Leave it for a few hours. It works for my paper which is quite thick. Good luck!