Hi all I've only done a wooden control cavity cover once before and it wasn't a success. Using Minwax Wipe-on Poly (or Tru-Oil, I now can't remember which) warped the cover significantly, I think because I only applied the finish to one side. The next finish will be a water based urethane (Enduro-VAR from General Finishes). I can see three possible solutions: 1. Spray both sides at the same time. Hopefully the expansion of the wood will be symmetrical and the cover will stay true. 2. Stick the cover to something flat to stop it from warping, then spray just one side. 3. Another technique that I haven't thought of! How does everyone else finish their control cavity covers? Thanks as ever for your help and insight.