Carry the chair outdoors or to a well-lit work area. Place a drop cloth on the ground and set the chair atop it. Examine each chair closely for any flaws such as splinters, dings, chips or missing slivers of wood. Sand extremely rough flaws with medium-grit sandpaper. Use fine grit on minor flaws and slightly rough areas.
Fill all holes and imperfections with wood putty. Apply the putty with your finger or a small putty knife. Partially smooth the putty with your finger, but allow enough extra putty to stay in place in case of shrinkage. Allow putty to dry completely per package recommendations.
Sand the puttied area smooth with fine sandpaper. Run your hand over the entire chair once more, feeling for roughness or other areas that need to be smoothed. Use fine sandpaper to sand away all remaining rough spots.
Remove the chair from the drop cloth. Shake the drop cloth out away from the chair. Brush sawdust off the chair using your hand. Wipe the entire chair with a tack cloth to pick up any remaining dust.
Set the drop cloth back on the ground and place the chair on it. Coat the chair with latex primer using a paintbrush. Include both sides of the chair back and the entire legs other than where they rest on the ground. Allow the primer to dry thoroughly.
Plan a whimsical design for your chair, such as a silly cartoon animal, funny sayings, amusing designs or any combination of these ideas. Draw the idea on paper to plan colors for each area.
Determine a main color for the chair and paint the chair that color using the acrylic craft paints. If one area of the chair doesn't need the background color on it -- for instance, a large sun on the seat of an otherwise blue chair -- skip painting that area for now. Allow the base color paint to dry thoroughly. Paint another coat if the primer shows through, or touch up any areas as necessary. Allow the chair to dry.
Paint the whimsical designs, sayings or artwork on the chair using various colors of acrylic craft paint. Use small brushes for detail work. Draw outlines with chalk as a guide if freehand painting is difficult. Allow each paint color to dry before adding another layer of color.
Seal the chair with polyurethane sealer. Brush it on with foam brushes using smooth, straight strokes. Spray clearcoat sealer can be used as an alternative.