our problem is that we have one machine with a rubber strip in it. The company that made it no longer exists and we cannot find a replacement strip. The other machine hasnt had much use in the past and is almost as good as new(aside form a little dust) It has no rubber in it. We have had the same thoughts on the tension being uneven. Its the only way to explain why the plier stretched canvases dont buckle. we have also noticed that our strainer/stretcher bars are terribly warped. I have been pulling the bad sticks and setting them aside to send back. Its almost as though the company we get them from doesnt heat treat them properly if at all. also the sticks themselves are finger joined in very close intervals. This used to not be the case. The joins used to be at about 3 foot intervals and now they are erratic. They can be 2 or 3 inches or they can be a foot apart. they tend to warp most consistently at the joints. Im not sure if warped moulding could be the cause of this but we are still testing different theories and are open to any and all suggestions. as long as you dont blame it on gremlins. we have exterminated all gremlins inhabiting the shop.
We too are having buckling or occasional sagging problems on Gallery wraps (about 5% of them). Get good stretcher bars to start with, not cheap ones, if you`re getting warped ones send them back to the supplier and get a refund. If they won`t take them get a new supplier. A good supplier will try and take care of their customers, just as you are doing. I don`t think the wood is the issue though in this case of sags. If it was stretched properly at first it ought to stay tight. The explanation that the wood moves is bogus, wood does move, but mainly laterally, not in length. If this was so then wood framed homes would be lifting and dropping all the time. Further the wood should be dry, and If not it will dry in a home environment and this will be less than 1 mm. on a meter. Not enough to cause this sort of problem I think. We use x braces after 24`inches. I like a tight canvas with no movement.
I believe the canvas is the culprit. When I get buckles they show up after about a month, in the customers home(s), so I spritz the back of it, and then put it under a radiant heater for a while, about 16 inches away from the heater. It shrinks and the buckles are gone. Still they can re-appear. A traditional approach is to make a stretcher (with pegs to expand the stretching frame), instead of a strainer-a frame with no adjustable (easiest to make). I cannot afford to do this step. It adds quite a bit of expense to the construction and as a sub contractor to galleries, I have fixed costs per item. My profit slides away to debt.
I`m wondering if the formulation for printing on canvas partially seals one side, then laminating is seals the canvas totally as well. Since the other side is porous it can `grow`and stretch by collecting minute amounts of water from the air. This is the back (inside) of the stretcher and it creeps from that point allowing the canvas to sag. With plywood, you always have odd numbers of veneers, as even ones warp. If we had a more even coating (both sides + the moisture problem I think we`d have less chance for stretch. We cannot afford to make stretchers as our profit margin is very tight, and must stay with strainers.
I need opinions on this as I`m thinking a light spray of fixative on the backside might stop this migration of water. All of our sags have been on what we call decor (good photos- gallery wrapped) with a limited lifetime. I have compared sagging with a competitor of ours and they don`t have sagging. None have appeared on paintings, and these are always mounted on a stretcher.
As for the rubber strip- try going to an industrial belting company, they probably have what you need. I found material for my Gapp stretching machine there.
