If you've already checked for mites, given your rat safe bedding, good diet etc., your rat may just need a weekly nail trimming to keep those sharp nails from causing more little cuts during grooming (the scratches from the rattie's own toenails can cause subsequent itchiness). Applying a bit of Neosporin (it's an antibiotic ointment) on any scabs can help prevent an infection. Here's a page that gives instructions on clipping rat nails: <http://www.rmca.org/Articles/qna2_99.htm> (it's the second of the three Q&As). You could also get some betadine (iodine) solution, and use about a teaspoonful in a cup of warm water; a soak for 10 minutes twice a day should keep the area cleaner too.