My first mistake was to not let the acrylic dry long enough. Although acrylic can be dry to the touch in half an hour that doesn’t mean it is cured. Curing is a loose term that means the paint is dry all the way through, even thick passages, and that the elastic quality of the film has diminished. Acrylic paint will always be more flexible that dried oils but acrylic paint that has not cured will have a noticeably elastic, rubbery quality.
|Sanding reveals the white undercoat|
Many factors make curing times variable like humidity, temperature, color of paint, brand, and thickness. In very dry climates it’s possible to cure acrylic in two days. But to insure quality sandability I think it’s best to wait three days (I live in Utah which is the driest state after Nevada). To determine if the paint is adequately cured press your thumbnail into a thick layer of paint. If it gives at all or allows the nail to easily leave a mark the paint is not cured. If a passage feels cool to the touch that means there is still water lurking in the surface below.
After three days the paint will have an overall tough, resilient quality and the surface is ready for sanding. For best results I use wet-or-dry 3M sanding pads (although I assume any wet-or-dry sandpaper will work). I wet the surface of the dry acrylic painting with a spray bottle with copious amounts of water. Once the water has soaked in a bit I begin sanding with the sanding pad (it’s like a tough sponge that has been coated with an abrasive). If the paint is properly cured you will begin to see colored moisture begin to flow down the canvas as the grit is washed away. I spray lots of water and use a vinyl raingutter with capped ends to catch the excess water.
I’ve had unexpected success with this method and have achieved some great textural effects. Even super tough films, like Golden’s Clear Tar Gel, can be effectively sanded if properly cured.
Let me know of you have any variations on this process.
Brad Teare –January 2016