If you purchased a painting on stretched canvas from me or anyone else, here are a few common occurrences that have quick and easy solutions:
Dent or Canvas Sag
Let's say you accidentally pushed against the backside of the stretched canvas painting which created a dent, or maybe the canvas has simple sagged over time due to temperature and humidity levels...
Spray the backside of the canvas with warm (not hot) water and as the water evaporates the canvas shrinks back into shape. Not DO NOT over-saturate. It is better to repeat this step several times in intervals of 1 hour, rather then over saturate it the first time.
Warped Stretcher Bars
Let's say when your painting was traveling from it's original location to you and was exposed by the shipping carrier to extreme heat, cold, and/or humidity OR over time in your own home, due to changes in temperature and/or humidity levels, the wooden stretcher bars may have warped. Sometimes the warping can be significant, most of the time they are slight and unnoticeable.
Solution for slight warps:
What you will need: Bowl, water, rag/towel, Kraft or Butcher paper, 4 books and 4 weights (either exercise weights or simply 4 heavy rocks)
-Pick a location in your house that doesn't get a lot of traffic and will be out of your way for a day or 2. Preferably the floor or a table.
-Create a clean dry surface where you will later lay your painting face down. This is important: DO NOT use a plastic bag since it can stick to a painting, DO NOT use a newspaper since the print can transfer onto the painting. Use clean non-stick material such as butcher or Kraft paper. If you purchased a painting from me, it already came with large sheets of Kraft paper that the painting was wrapped in (you should keep those for just in case)
-Fill up a bowl with luke-warm water. Turn the painting over, place it face down on the surface you have just created, and wet the wooden stretcher bars with a drenched rag/towel, making sure to keep wetting the bars until you see they are sufficiently wet. Place 1 book on each of the 4 edges of the canvas and then place your weight on top of the books. Wait 24 hours. For good measure, I recommend taking the books and rocks off after 24 hours, rewetting the bars, and placing the books and rocks back for another 24 hours.
IF this does not work, it means the warp is a significant one and the painting must be taken to a local stretching and frame shop to be re-stretched.