This Masterpiece Tight 'N Up 4oz Retensioner once applied, soaks into the canvas' threads and fibers causing them to swell and contract to tighten the canvas. As the water evaporates, the binder agents remain in the fabric material and lock the fabric into the contracted state, restoring the tension and leaving the surface flat and tight. This product will not harm the canvas, acrylic gesso, or acrylic and oil paintings. It is not recommended for use on polyester-based canvas, oil primed canvas, inkjet printed canvas, or Giclee prints. It is made of archival acid-free materials and dries clear. It can be used either before or after painting and can easily be applied under the stretcher bars with a brush.