I live near Seattle, so the "plastic sandwiched between 2 sheets of glass, left in the sun" method doesn't work for me. 
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Instead I preheated my oven to 175F, with a large, empty cookie sheet on a lower rack. I put the warped plastic between 2 sheets of tracing paper IMG_1366 (resized).JPGon top of another cookie sheet, covered with a flat piece of plywood and weighted down with a 5# dumbbell. IMG_1367 (resized).JPGThat all went on a rack above the lower shield.IMG_1369 (resized).JPG Then 30: with the oven on, followed by 30: still in the oven, burner off. Finally, slow cool on a bread board until the weight could be removed with bare hand. Result: flat plastic with little stress, independent of weather conditions.IMG_1370 (resized).JPG
Next, I gotta' figure out a way to bleach out the yellow - ideally weather independent, but....
Allentown, PA
