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Since the Pacific HVDC Intertie was commissioned in 1970, measures have been taken to prevent pollution flashovers of the outdoor insulators at the Sylmar West Converter Station. In April 1991, a research program was started with a view to finding the optimum type and layer thickness for different insulator coatings available at the time. Various types and thicknesses of insulator coatings and one type of massive silicone rubber plate were installed, together with bare porcelain plates as references, on test racks at the station. Annual inspections were performed the first three years and the last one after 10 years of exposure. The following conclusions may be drawn from the research program. Most plates coated with silicone grease of three different types and in three different layer thicknesses were still water repellent after 10 years of exposure. The massive silicone rubber plate and one out of three RTV coatings also showed good performance, while the other two types of silicone rubber coating had become wetter. Silicone greases performed better than other coatings in this test. A thin layer of coating, e.g. 0.5-1 mm, is sufficient for good long-term performance.