Fox Collectors

I am in the process of cleaning up a A grade stock and action from a gun that has a good amount of case color but some is covered with old yellowed varnish. I would like to hear suggestions on how to remove the yellowed varnish from the action without compromising the remaining case color?
Thanks

I've had good luck with acetone. It removes about anything and doesn't hurt the case colors.
Dave

I use an ultrasonic cleaner from Lyman. Might take a few 8 minute cycles but they work real slick.

Thanks guys!