The slots in the drying rack are just over 1/8" wide and cut at a slight upward angle. Painting boards must be 1/8" thick to be used with the Drying Rack. If you already have painting boards you can check them for compatibility by placeing a stack of 16 painting boards on a table and measuring the height of the stack. The stack must be exactly 2" high. If the stack measures less than 2", the boards will sag when placed in the rack. More than 2" and the boards may not fit or will be slanted upward when placed in the rack. Be sure to press the stack down at the edge where you measure to get an accurate measurement. Measuring a stack of 16 is suggested because that's about the minimum needed to get an accurate measurement with a regular ruler.