Heat gun first. Bend it the opposite way. Heat on both sides. Keep at it. Leave it alone for an hour or two. Look at it again... Repeat as necessary. I've found it better, sometimes, to leave a few wrinkles in the covering. Heating out the wrinkles can put the warp back. It's also possible to put slits in the trailling edge, bend opposite warp, and run thin CA. When it flashes off, you've got a mini spar soaked into the trailling edge. It helps. Patch covering and go fly. Do it from the bottom. It won't look too bad. Makes the ARF look right. Kind of. Patched together and improvised. Helps it to be stealthy. Beside you've marked your plane. Trim tabs work to even things out and compensate for aerodynamic anomalies that show up as you fly the pattern. Bawsa warps. A wing can come out of a jig dead straight than warp weeks later. I web trailling edges and mainspars, in part, to keep out the WARPS. Probably there are lots of ARFs and kit builts out there that have wings, stabs, fuses out of kilter. Depends on how close you look. At Brodak's a gentleman was selling an ARF that had just won Intermediate. Was it warped. Let me count the ways. Would be nice if experienced builders talked about how to keep out the WARPS and compensate for warps that show up. How do people age their BAWSA, do they. etc. It's possible to ruin a few hours obsessing about warps that suddenly appeared on pride and joy. Usually means time to stick on THE TAB and GO FLY.