Keep doing it until entire mat is flat and covered with heavy books.
How to flatten drafting mat.
Fight curl with curl unroll the mat as flat as you can get it.
Cut 4 strips of self adhesive linen hinging tape.
Creating this opposing new curl shape will in effect straighten the curled plastic.
Place your mat on a smooth flat surface outside in the sun on a warm sunny day until it flattens check it every 15 minutes.
Leave the mat between the metal sheets until they return to room temperature.
I was able to straighten it flat again by laying the mat on a towel on a table covering it with another towel then using a steam iron on the lowest temperature at which it would still steam ironed over the towel and mat.
Now your mat is ready for installation.
After it relaxes and is flat move it to a flat cooler area in the shade or indoors until it cools and becomes more rigid.
But i think there is a solution here this might seem counter intuitive but one way to get a mat to lay flat again is to add back a little heat.
If your paper is less than 30 inches 1 4 inches is typically fine.
Remove the paper backing from one of the strips and place it in the top corner vertical with the sticky side up.
What about heating areas with a hair dryer and while it s still warm plop some heavy books on it.
Next roll your map or poster around the tube with the image facing out.
Roll the mat back up and read the stop and read.
Keep the same pressure throughout your cut and be very careful not to dip out or in from the penciled line.
Make your first hinge.
To flatten a rolled map or poster start by unrolling it facedown on a flat surface and placing a cardboard tube at 1 end of it.
Pull the razor slowly and steadily towards you stopping at the bottom corner.
I was careful that the mat didn t get too warm.
This will soften it down and then you can try setting something heavy on it to get it to retain its flat surface.
Then run your hands over it to make sure there are no bumps lumps or other flaws to telegraph through.
Now roll your mat back out.
Use a straight edge razor to cut the backing and mat boards.
Then flip it over and reroll the mat.
Even if you think your floor is clean sweep it one more time.
Not what you want to have happen to your expensive cutting mats.
Press the razor down firmly at the top corners of the boards.
Once the item is wrapped around the tube image facing out place rubber bands around it so it won t unravel.