Place a spray painted image in a scanner and scan the image at 300 pixels per inch using the grayscale setting. Load a laser printer with clear vellum paper.
Open the scanned file in Photoshop and select print from the file pull-down menu. From the print dialog box, select "screen" and enter "55 lines per" inch in the screen frequency field. Close the screen box and finish printing. This print is a film positive and will be used to image a screen printing stencil.
Wash a screen printing stencil with soap and water. Rinse thoroughly and let it dry overnight. In a dark room, coat both sides of the stencil with photo emulsion using a squeegee or stiff piece of art board. The result is a screen-print stencil coated with a thin opaque layer of emulsion. If the coated screen has drips on it, use the squeegee to remove excess emulsion. Place the coated stencil face down in a dark area to dry.
Cut a piece of foam rubber to fit snug inside the screen-printing stencil frame. In the dark, place the foam rubber on a flat surface. Place the stencil face down over the foam rubber. Put the film positive face down centered on the back of the screen printing stencil. Put the piece of thick glass over the film positive.
Hang a halide light 18 inches over the glass and turn it on to expose the image on the stencil. Turn it off after 10 minutes. Develop the image with a spray of warm water to both sides of the stencil. After the image appears, continue spraying until all the detail of the spray painted image appears. Blot both sides of the stencil with newsprint and set in a well lit place to dry.
Place the imaged stencil over the item to be printed. It could be paper, wood, art board, fabric or a T-Shirt. Put some screen-printing ink in the end of the stencil opposite yourself. Use any color ink you like. Pull the ink towards you with the squeegee applying firm pressure. Pull two strokes to insure a good print. Lift the stencil frame to check your print. Multiple prints may be made. Clean the ink out of the stencil after printing.