I must have gotten the last of the "Spotone" branded spotting dyes from before they went out of business. The new stuff is marketed by Marshalls now, and is called "Spot All." Whatever. This stuff lasts a long time because a very tiny bit is all you need. Water it down a lot, lighter than the lightest tone you'll need and then, like 2F/2F wrote, apply it in tiny little spots, little by little, until the spot is no longer obvious. Over do it and it gets really ugly, really fast. Fortunately, you can reverse it. Simply soaking the print in clean water will remove the dye in 10 minutes or so. Of course, you need to dry it and start over. Resin coated papers are tricky to spot, since they repel water from everywhere except the gelatin. Spotting fiber based papers is, by comparison, a piece of cake.