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What Causes Sticky Varnish?

Ask the Expert: “I varnished a painting several days ago using retouch damar and it's still sticky. Will it eventually dry? What could cause it to stay tacky?”

A: Usually when varnish remains persistently sticky it's the result of application in a humid or cold environment. Sticky varnish can also be caused by too-thick application, or by re-coating an insufficiently dry layer.

Traditional varnishes made in-studio (e.g. damar and mastic) are most prone to stickiness. Even factory-made products can also suffer from application when it's humid or rainy, so it's a good idea to avoid varnishing unless conditions are warm and dry.

Most of the time, residual stickiness will resolve itself but depending on thickness and condition of the coating, it may take an impractically long period of time. (Just maintaining a dust-free surface for more than a few hours can be challenging.) If you've waited a very long time (more than 2 weeks) and you're confident the paint is completely cured but still has good flexibility, removing the varnish and re-coating may be the best approach. While removing the defective layer, make sure to support the canvas from behind, and use stiff cardboard "shims" between the back of the canvas and stretchers. If you must reapply varnish in humid conditions, try a synthetic product in place of damar- acrylic and ketone-based products often dry quickly to a tack-free film when conditions are a little damp. If it's actually raining, though, it's generally better to just wait for another day. After varnishing, allow the painting to dry thoroughly in a dust-free environment that is warm and dry with good air movement.

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