While your acrylic gesso or oil ground should tighten up your canvas, various factors such as humidity can cause your canvas to loosen a bit. If you’re experiencing this, you may want to try some of these tips.
Spray the backside with water. Using warm water in a spray bottle, gently mist the backside of your stretched canvas. It should dry within a few minutes and tighten up your canvas. Be aware though, you can only use this method five times. After that, it no longer works and may cause your canvas to slacken even more.
Use canvas keys. If you’ve purchased a canvas before and wondered what these things are:
They are canvas keys or tightening keys. If you’ve built your own stretcher bars, these won’t work. They will only work on purchased stretcher bars, as they have slots in the corners where these keys fit in. Insert two keys in each corner and knock them in gently with a hammer.
Here’s a quick video demonstrating how to use canvas keys to tighten your canvas: