How can I clean my rats' tails?

I bathe them about every 4-6 weeks or so but I can never really seem to clean their tails very well. I use oatmeal puppy shampoo so that it doesn't irritate their skin and I take a damp soapy cloth and wipe their tails from base to tip, going in the direction the scales go so I don't hurt them, but they always seem to look and feel filthy! Any tips?