One issue pastel painters often confront is the flatness of their painting surfaces. Many papers and surfaces are made from cotton rag for archival reasons. As superior as these are, they’re also prone to wrinkling and warping when moisture is introduced. Depending on your technique for applying pastel, this can be a major or minor inconvenience. Artists that employ a stroke application with pastel are less bothered than those that swipe the pastel stick across the surface. Often this swiping technique requires the pastel stick to remain in contact with the surface while pressure is varied for effect. If the paper is washboard or irregular, the pastel hits the high points with more force. This deposits a heavier mark, creating a speckled appearance.
The weight (or thickness) of the paper as well as mixed-media techniques employing water can exacerbate the wrinkling situation. The heavier the weight of the paper, the less prone to wrinkling and warping it will be. Thin papers, like most charcoal/pastel papers, should be kept as dry as possible or mounted to a rigid substrate in advance of painting. Heavy rag papers, like watercolor papers and some etching papers, can be stretched prior to painting. Follow good watercolor painting procedures for stretching these papers. Sanded papers like Wallis and UART are manufactured on rag or pH neutral papers that are very prone to wrinkling when wetted. For this reason, many artists who work with watercolor or spread pastel with water to produce an underpainting have their papers mounted in advance (see my blog posts on January 14, 2008 and January 21, 2008).
Kitty Wallis, in the literature that accompanies Wallis paper, describes a method of reverse taping and stretching the paper taut when wetted, which allows the paper to dry considerably flatter: Before wetting the surface, turn it backside-up and run a thick strip of tape half over the edge. Flip the paper over (right-side-up) and place it on a secured drawing board. You’ll now have a portion of exposed tape with the tacky side facing out. Run an additional strip of tape around the paper adhering to the tape edge and drawing board. Once the paper is wet, the tape can be lifted and the paper gently tugged tight and then reattached to the drawing board. Some methods of mounting paper rely on a moist glue to adhere the paper to the mounting board. These are often prone to bowing once dry. If you’re producing your own mounted paper using this method, make sure to place it under heavy weights overnight or until it has had a chance to dry. This can alleviate most of the bow. If it persists, you can try painting the back of the mounting board with an acrylic varnish or paint. Often this will shrink as it dries, pulling the boards back to a flat condition. Another method is to mist the back with a fine spray of water. Place this under heavy weights while wet and allow it to dry. The best way of avoiding bowing is to use a dry-mounting procedure. It may cost a little more but usually produces the flattest outcome.