Does anyone know how to fix warped stretcher bars?

I did a large commission piece, 30 x 50 inches, about a year ago on heavy duty stretcher bars. While it was hanging in the clients home it warped. I took the piece and held it flat and installed heavy metal corner braces on it, to no avail. Still warped. Does anyone know how to fix this without re-stretching it?

I have a large stretched canvas which warped. We fastened a sheet of MDF on the back which helped but it is still a bit warped. I think the answer might be putting the painting in a float frame. I use these for exhibition work and it keeps the stretchers flat with being screwed down on all sides.

Thank you Anne. That does seem like the way to go. Just hope they’ll go for it. Would sure save alot of trouble.

Out of interest I Googled this subject and it appears a lot of people experience warping particularly on larger canvases. One tip was to wet the wood on the back with the painting face down and place heavy weights on all four corners until it dries. I might try this as an experiment on a canvas I won’t be selling. I also saw a Youtube video about fitting together aluminium stretcher bars which won’t warp. Might be more expensive but for an important commission it could be worth the investment.

Yep, tried that wetting thing. Didn’t help and your right it was kinda scary doing that. Something to try on a piece you haven’t sold maybe.

I suspect you will never get it flat without restretching in my opinion. You didn’t mention whether there were cross braces on those heavy duty stretchers. I would recommend a cross brace every 30". Your canvas should have at least one on the long side.

The painting may also begin to show ripples in the canvas possibly from uneven tension while stretching which I’m guessing might be part of the issue. I have taken the warp out of some canvasses when they are framed. The frame can add more rigidity but not always. I’m guessing yours may not be framed.
I would restretch on stretchers with at least one cross brace, stretchers with keyed or mechanically adjustable corners.
Some of the nicer stretchers I have seen are from
I have stretched a canvass that was 4’ x 20’ with cross braces using custom made stretchers with the mechanical keying system. They are awesome but not cheap. You get what you pay for.