From your description, it sounds like the stretcher bar is warped. There's no way to un-warp a bad stretcher - you'll have to buy or build a new stretcher frame and re-stretch the painting. First, remove the staples or tacks that are securing the canvas to the stretchers. Be very careful with the unstretched painting - it could crack, especially if it's an older painting. Build or assemble the new stretcher frame to the exact dimensions of the old one and make sure it's not at all warped. Finally, gently stretch the painting over the stretchers and staple the painting to the frame. Make sure it's not under too much tension, though.
Stretched and unstretched hundreds of paintings
· 8 years ago