Gates does not recommend applying dressing to belt drives. While belt dressing may temporarily quiet a slipping V-belt drive, it only masks the real problem (i.e. low belt tension). If a belt is slipping, it should be re-tensioned.
Additional information on drive inspection and troubleshooting is available in the Belt Drive Preventive Maintenance & Safety Manual at http://www.gates.com/brochure.cfm?brochure=1224&location_id=3288/.