Something that needs fixing

[Locomotive]

Okay so I have this 5 ft x 4 ft wooden frame which has been virtually unused for more or less a year. I haven't dabbled in painting for quite some time now but just recently I had this urge to give it another go. I took the frame out, mounted it, and of course primed it. Much to my dismay, a day or two after priming, I noticed that it's slightly warped on one side. It just won't lay flat. I don't know if too much moisture from priming was the culprit, or did I just put too much pressure on the wooden bars while I was stretching the canvas.

I tried to fix it with the method I know--laid it on the ground, did a bit of sprinkling, and then I placed four heavy boxes on all four sides. Unfortunately after leaving it for more than 48 hours, the problem is still there. I'm not totally new to this, I've primed, stretched, fixed, and painted huge canvases before. But this one's pretty stubborn.

I hope you guys could help me on this one, is there any other way I could fix this problem without dismantling it? Stretching such huge canvases is quite a laborious task and I really don't prefer dismounting this one just so I could make adjustments on the wooden frame. I would do it tho, but only as a last-ditch effort. And gesso is pretty expensive, I'm afraid that if I dismount and re-stretch the canvas some areas would crack, and I would have to re-coat and sand the surface once again.

Also, while I don't prefer frames on paintings as big as this one, would framing be able to fix my dilemma? I'm thinking just painting on it and simply frame it afterwards just to correct its posture.

I've researched for possible solutions already and this thread might be redundant, but since we're dealing with a large canvas here (which might require different fixes as opposed to the ones that are of smaller scale), I decided to make a new topic about it.

Thank you guys in advance. Maybe I'll take some photos for reference when I have enough free time, in a day or two maybe after my workdays. Have a great day ahead!

[DaveMak]

Framing definitely could correct the problem but depending upon the amount of warp you cannont depend on anything other than one big honker of a frame to flatten the strainer.