This paper addresses the long-term aging of silicone rubber shedded insulators mounted horizontally, and specifically relates the experience from a highly monitored six-year trial of two composite cross-arms designed for 400 kV. Weather conditions and leakage currents were continuously monitored in this moist and temperate area. In addition, the growth of algae on the surface over time is reported. As expected for the coastal location, strong seasonal variation was seen in leakage currents and this is associated with wind direction and resulting weather variations. Leakage currents remained low but increased linearly with time. Despite apparent large areas of algae growth on upward-facing surfaces, and some associated loss of hydrophobicity on some surfaces, the leakage currents on the insulators continue to be satisfactorily low. Cleaning one of the insulators after five-and-a-half years temporarily returned the current to its original value. In the short-term, leakage currents were heavily dependent on relative humidity, and peak currents are shown to be strongly dependent on the length of preceding periods without precipitation. It is suggested that measurement of these variables may be used to evaluate surface condition and be an effective asset management tool.