The most common issue where the sag occurs across the canvas weave. Example: a printed 24”x36” canvas from a 36”roll, with the weave going across (horizontal), will tend to sag in the 36” direction. So take note of the direction of the weave when framing. Rotate the print 90 degrees and print the short edge across the roll.
Staple the Long Side First
Staple a clean line on the longest side and then move directly across to the other parallel side and staple a clean line. Example: for a 24”x36” print, start with the 36” side, then move to the OPPOSITE 36” side, not around the edge of the frame.
Add Cross Bars After Wrapping
A good way to avoid sag is adding crossbars after the frame is fully wrapped, stapled, and glued. Also, adding corner braces after framing will add more tension.
Use Archival Glue
Put a bead of archival glue on the stretcher bars as you set the edge on the frame. This technique works for stretcher bars with an inset lip.
Work in a High Humidity Environment
Working in a high humidity environment allows the canvas to absorb this humidity and relax more. This will give a tighter stretch when the canvas is moved to a drier environment and shrinks again.
Consider Making Your Own Stretcher Bars
Most commercially made stretcher bars have very poor corner joints. These joints tend to move and cause dips in the corner of the canvas. If you insist on using store bought bars, only buy the heavy-duty bars and glue them together first. Sand the corners well and make sure you do not round them. Avoid using keys in the corner slots.
Here is a suggestion to correct the sag after the print is stretched on the frame.
Wet the Back and Let It Dry
Dampen a sponge and rub it across the back of the canvas or use a spray bottle to moisten the back. Allow it to dry on its own and it should tighten up on the frame. You can also try a hair dryer blowing on the damp back of the canvas to speed up the process and tighten it.