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If you want very flat glass then go with a mirror. But like others said try the other side. If you get frustrated check out the EZABL system as it can compensate for bowed glass. I run a company called TH3D and its our auto bed leveling product for the CR-10 and other machines.

As for favorite surface I use the TH3D EZMat on all my machines. We just got back from MRRF and we sold a lot of them. We got a lot of feedback on the surfaces from the CR-10 community and at MRRF. Let me know if you have any questions.