IT IS NOT A LOST CAUSE!
Your warped stretcher bars on your larger canvases may or may not be because of how you stored them or that you bought them that way BUT who cares? They're warped. And you might not have noticed until you finished the painting and hung it on your wall, either for yourself or to sell on your site or in a show or worse yet, it's off to a gallery! OH NO......not the gallery!
Relax. I know the solution to fix your warp. Not 'your' warp -LOL- but the warp of those dang stretcher bars that make your painting hang funny on the wall.
((this painting is for demonstration purposes only)) but trust me, I've had plenty of warped stretcher bars.
So the cure is simply as you can see from the above photo. All you need is a spray bottle full of water. Some weight, my small dumbells worked just fine, along with a full laundry soap bottle.
Lay your canvas/painting face down on something protective such as craft/kraft paper or gator board but something so the canvas is protected whether there is a painting on it or not. Spray the stretcher bars with water. Now apply the 'weight' to each cover. I generally start this in the late afternoon so that by the time I'm ready for bed I can spray the stretcher bars again.
Leave your canvas for minimum 12 hours. You'll be shocked and amazed. When the weights are lifted that your canvas will now lay flat against the wall. And everyone said YAHOO.
Good luck to each and everyone of you!
Feel free to leave questions or comment or tell me all about your success.