I have a "plastic" desk pad that has been lying on a shelf for a few years. It was rolled up and the roll has flattened over time from its own weight. Now the pad is "wavy" and no longer lying flat:
How can I get the desk pad to lie flat again?
Putting some heavy books on it for a few days didn't do much difference. My guess is that I would have to bend the pad further than flatness to counter the waves, and that is difficult to achieve precisely and might result in further, more complicated waviness. Also, just flattening the desk pad using weight might take more time, but I'm impatient and don't want to wait weeks or months.
I also thought about using a flatiron at a low temparature, but I'm concerned that I might damage the desk pad. I don't know what material it is made of.
I tried what @Syed recommended in a comment: "Roll it length-wise and leave it for a few days..."
I left the desk pad standing upright in a corner for a week:
After that, the long edge appeared straight:
The desk pad had aquired a strong bend along the short side, but this was uniform and not wavy:
I put some books on the mat for another week, and the bend along the short side has now disappeared, but as you can see, the original waves along the long side are now visible again, although less pronounced than in the beginning:
All in all the desk pad lies flatter now than before, but it is still not lying flat enough to write or draw on. The waves, although they appear slight, are resistant enough to be an obstacle to the movement of the pen.
All in all, @Syed's suggestion made the desk pad flatter, but not flat enought to be usable for me.