Unfortunately it does have a grain and shrinks more in the length direction than in the width. I was trying to skimp on MonoKote and thought that being plastic there would not be a difference in shrinkage. I didn't save anything as I had to do it over (the more heat I applied the bigger the sag between ribs).

Then I remembered my engineering background, it is caused by the way it is rolled between rollers during manufacturing (sheet metal does the same thing and causes problems in stamping because it stretches different).