Create two solids. The bottom will be your stencil and final painted image, the top will be the spray paint. Create a mask the shape of our stencil. Duplicate the layer and change the mask mode to subtract. The subtract layer is your stencil and the add layer is your resulting painted image. You probably want to change the color of the stencil solid. On the third solid layer above use the Paint tool (set to the color of your painted image. In the paint panel set duration to write on use a feathered brush and cover your stencil with paint. In the Effects panel Check the paint on transparent box. Pre comp the paint and stencil layers. Pull back the stencil/paint comp to reveal the final image. (add a little paint drip with the paint tool).
Hope that’s what you were looking for.
Rob