The Bepi Colombo spacecraft was successfully launched on 20th October 2018 and is now on its way to Mercury. The materials and coatings on the Mercury Planetary Orbiter (MPO) and Mercury Transfer Module (MTM) solar arrays will need to withstand a prolonged extreme environment of high-intensity UV radiation at high-temperature (HIHT). The solar arrays must be gradually off-pointed at close sun distances in order to avoid over-heating, up to an angle of over 70° at 0.3 AU for the MPO. This means that some of the materials and coatings, as well as the solar cells, will always be operating reasonably close to their temperature limits. Extensive on-ground testing was performed to qualify the solar arrays for the BepiColombo mission environment and during this testing some materials and coatings were pushed beyond acceptable performance limits. In this paper we give an overview of selected test programmes related to the solar array materials, and we present results of experimental investigations which provide some scientific insight into the likely causes of the materials degradation observed, and the lessons learnt in the cases where over-testing had occurred. This included testing on bare and bonded optical solar reflectors, solar cell cover glass and adhesive, and the solar array edge shields.