Wow. Lots of good advice here.

Unfortunately I live in an area that substrate supplies are almost impossible to get. I am an hour and a half from both Houston and Austin, TX. The ONLY supplier is Reese supply but they carry a very limited (like 5 sheets) of any brand of HDU. No sign supplier or Lumberyard within 3 hours of me carried MDO. Spend a full day calling around and searching the internet. Plenty of Dibond type products and aluminum products are available.

Like I posted earlier I have only had experience with Signfoam, I'll have to go back and research the forum on bits, speeds and properties of the substrates I do have accesss to.
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So...after reading all the advice, I purchased (2) sheets of 4x8 of 1" PVC. The sign is 4'x4'. I realize it will be heavy.

I have planned to have the sign hung on the barn side with two large aluminum French cleats attached both on top and bottom.

My question is... Is the 1" PVC AND the support of the aluminum cleats enough to help prevent warping? If I add a backing I'm adding to the weight.

All alternant suggestions are welcomed and appreciated. Thanks.