Rick:
The best, sure-fire way is to have the frame sandblasted, however that can be rather pricey and requires the body to be removed. Spray the pockets real good with Simple Green or Purple degreaser. After is has soaked, most of the matter will dislodge. After the matter is removed, give it a good coat of POR15 or other rust inhibitor/encapsulator/preventor.
Tony
