An artist’s stretcher can be expanded to tighten a loose canvas. An artist’s strainer cannot be expanded to tighten a slack painting. That is the only functional difference between the two major types of painting supports. But this is the point where all the trouble begins: whether a painting support expands or does not expand has limited bearing on its underlying structural integrity. The straightness and strength of the wood used to make the stretcher bars and crossbars determines the painting’s end result flatness.
Selecting wood to make your strainer will take the greatest amount of time. You need to hand-pick each and every piece. ‘Sight’ each length of wood to see if it is twisted or warped. Look at the length of wood ‘end-on’ to select straight lengths. When you have chosen what appears to be useable wood, lay the wood flat against a level floor to double check. If the length of wood ‘wobbles’ as you try to rock it from side-to-side, reject it. It’s not flat and therefore, will never make a flat strainer. (One bad piece of wood will ruin the three good pieces and you’ll end up with a warped painting.) With good wood you can build a flat strainer with little skill; but with bad wood you won’t be able to build a flat strainer with great skill. The secret is all in the wood.
You will need a flat work surface to assemble your mitered stretcher bars. Even your flat lengths of wood will have a slight bow or ‘crown.’ Lay your bars on the work surface with the crown side up: this will cause the four corners to make contact with the work surface (and eventually the wall when the painting is hung.)
The secret to strainer construction is to not begin by nailing or screwing the mitered corners together. Doing this invariably throws the strainer out of square and twists it out of flatness. Instead, begin by applying wood glue to the mitered surfaces and join the corners with firm pressure, drawing them together with masking tape wrapped around the outside edge to hold the joint together. Use a tape measure to take a diagonal measurement in both directions to ‘square-up’ the strainer. As it lays flat on your work surface, the freshly glued and taped-up strainer can be gently squeezed diagonally until you get the same measurement in both directions. If it jumps out of square, put weight on one corner to hold it until the glue sets.
Allow the glue to dry overnight; rushing this technique will compromise your results. The next day, pre-drill and screw together the mitered corners from the outside edge. It is not advisable to hand nail mitered corners because the pounding action may crack the glue joint (wood glue has poor ‘shear’ strength).
To add a lip to a strainer or stretcher bar, glue and nail lattice stripping around the outside edge of the strainer. This method will increase the strength of the strainer to resist warping. (The usual method of nailing quarter-round molding on the surface of the strainer does not increase the strength.) Lattice stripping can be purchased in a variety of widths and thicknesses to accommodate the size of your strainer.If a strainer is larger than 30” you may need to add a crossbar. Cut the crossbar to fit snugly and glue it in place with ‘crown’ side up using a pipe clamp to secure it to the strainer bars. Once again, allow the glue to dry overnight.
Executed properly, this procedure for fabricating a strainer will create flat and strong strainers every time. It is time consuming and tedious to make good strainers. And it’s understandably a process painters hate to waste time on; you just want to start painting ASAP. Who wants to spend an hour selecting wood? Who wants to make a dusty mess in their studio cutting wood? And I’ve seen very few studios with large flat work surfaces, which are essential for making large strainers. What’s a “pipe clamp,” anyway? Everyone wants to paint on a perfectly flat and square canvas, but few want to spend the time and effort making one. What to do?
Stretcher bars can be bought at art stores in standard lengths. If you follow the wood selection instructions I’ve presented, you will get flat and perfectly square stretchers with art store quality stretcher bars. By putting a drop of glue on the finger-jointed miter when you assemble it, the stretcher will stay in square when you stretch the canvas. Later, if it needs to be keyed out using the wooden keys, the drop of glue will shear loose and the stretcher will expand in the normal manner.
Custom made strainers and stretchers are also available from many companies if you need lengths not available at art supply stores, heavy duty stretchers requiring many crossbars, extra deep profiles, or irregular shapes. They will cost twice as much as making your own.
Custom made strainers can be extremely strong if they are joined together with dowel pins, corner triangles and lap-jointed crossbars screwed together on both sides. The strongest stretcher bars are fabricated from two pieces of wood: the lip and bar. This system creates a stretcher bar having the increased strength of an I-beam. Stretcher bars milled from one piece of wood will not resist bowing. Stretcher bars fabricated like an I-beam will be difficult to bow even with tight stretching.
Expandable stretchers with mechanical hardware are widely used. They were developed in the 1950’s by James Lebron when he employed a type of fastener used for formica counter top installations, permitting long coffee shop counter substrates to be seasonally adjusted to prevent cracking of the formica surfaces. Mr. Lebron’s use of the patented Knape & Vogt® Tite Joint Fastener for stretcher construction was presented by the Museum of Modern Art as an innovation to expand large-size post war paintings.
A major conceptual flaw mars this ‘innovation.’ Tite Joint Fasteners are designed to draw joints together, not to expand them. A better solution is to use the “ball” component of the Tite Joint Fastener and mate it with a “hanger bolt” (instead of the “tightening bolt” and “locking ring” provided by Knape & Vogt). This eliminates the flaw in the original design. Now the joint can be expanded and have mechanical strength by virtue of the hanger bolt being securely screwed into the stretcher bar. (By contrast, the “tightening bolt” provided with the Tite Joint Fastener floats in an oversize hole imparting no strength to the joint when it is expanded.) While Mr. Lebron’s ‘innovation’ was ingenious, the Knape & Vogt Ball/Hanger Bolt method I’ve described is the only way this hardware should be used for fabricating mechanical stretchers.