You might try first just to re-stretch the the 1/4 of the painting that has the problem, it might be enough. I have often found that hot water won’t be enough either, it looks tight for a day or less then sags again. By the way, this doesn’t make the corner any weaker, there is still a full dovetail join inside the corner. But if you have painted heavily on the sides that sounds even more challenging, as your canvas will now hold a box shape even without the bars and it will be hard to get that to lie flat even for the few mm you need to pull over the edge. A slight dip or sag in the corner can become the centre of attention, I have found, it can be what the viewer ends up focussing on for some reason.

I have personally found this a very difficult task where I wet the bars and dry the canvas under weights and it may still re-warp later. If you expand by tapping the bars it is hard to over-tension the canvas, it only goes as much as it can. I think it is because when I hammer on the centre of the bar it moves both ends of the bar the amount that they will freely go, which might be slightly different to each other. Attach the jig to the mitre gauge where the thickness of the wedge is determined by the distance between the stop block and the cutting line of the blade.Of course you could un-wrap the corners and remove the staples, and as now stretched, the canvas will not distort and you could then push the wedges in for more tension. Apparently the reason the canvases are stapled in this way is because the stretchers are put together first before the canvas is stretched and it prevents the rectangle from moving and becoming a rhombus before the material is attached. This is what is meant when someone says you need to ‘bang your stretchers apart’, they mean to tap the stretcher bars in their centres, one at a time, away from the middle of the canvas. The keys (wedges) are not designed to force the corner overlap open, but rather to sit in the gap to keep the tension as the bars try to close the gap. Did you use a mount/matte or a slip to give space between the painting and the glass or is the painting up against the glass?

Please include what you were doing when this page came up and the Cloudflare Ray ID found at the bottom of this page. Bashing it could split the wood of the wedge, or you could bounce off and hit the back of the canvas, or over-stretching of an oil painting could crack the brittle painted surface. Claire, our former bespoke canvas maker, explains: “If you need to tighten a canvas during painting, take a slip or block of wood, hold to the inside of the stretcher bar and tap this out with a light hammer (the wood stops you getting hammer marks on the stretcher bars) do this on all four insides, then push the wedges home, they are there to act as keys to stop the stretcher from contracting they are not to be hit with a hammer. Simply screw a square block to one corner of a plywood base and add two straight strips to the opposite outer edges of the base.

Vise racking stop: Instead of spacers, I use a wedge with a lid to support the open end of the vise to keep it from racking. From the back of the canvas slide a wedge, point first (top of the triangle), into each slot, one at a time. I came up with a cutting jig that is unlike most wedge-ripping jigs; it is adjustable to make wedges of different slopes and sizes on the table saw.