How can i straighten a painted canvas?

My wife and i bought a painting, about 4 by 2 feet. I am building a floating frame and just noticed from lining up the rails that the top of the canvas is bowed. Meaning that if i position about 1/2 inch gap for the floating frame on the two sides, the middle of the rail is touching the canvas (when there should... show more My wife and i bought a painting, about 4 by 2 feet. I am building a floating frame and just noticed from lining up the rails that the top of the canvas is bowed. Meaning that if i position about 1/2 inch gap for the floating frame on the two sides, the middle of the rail is touching the canvas (when there should ve a similar 1/2 inch gap. Hope that makes sense. I don't know if there is a way now that i can try to straighten sehow, even though the painting is done. The painting is oil.

Thanks fir any thoughts you might have.
Tim
2 answers 2