My recipe is 50% mineral oil, 50% acetone (similar recipes and variations on the theme are also out there and are no doubt equally effective), scrubbed on with an old toothbrush or a soak overnight for extreme cases. This stops it in its tracks and protects against any further lead rot, but obviously can't reverse existing damage. However, this gives the model an oily sheen which makes it difficult for paint to adhere to, so before painting a quick scrub in warm soapy water will help remove the surface oil.