Keep the black negative opaque. When you have the money buy a Winsor Newton series 7 sable watercolour brush size 00 from an art store. Another brand would work but these are the best and well worth the money; about $14. When you have clear spots on a negative or scratches, Use the opaque to cover the spot. A white spot on a print is easier to disguise with Spotone than a black one. The brush will also be the best for print spotting. Use it with respect and care and it will last you many years.