Spot Color Gradient Film Saver
If you're creating a gradient using a spot color that fades to white, to make sure your gradient appears just on the spot separation plate, create the gradient to go from the spot color to a 0% tint of the same spot color (for example, go from 100% red to 0% red). Just click on the color's opacity stop (that appears above the stop in the color ramp) and then lower the Opacity in the bottom-left corner of the Gradient Editor. That way, when you do your seps, the entire gradient will appear on the red separation.