This is the result of further experimentation with using clear silicone sealant as a protective layer on the solar cells of garden lights to try and prevent water ingress without attenuating the amount of light reaching them.
It seems to work well. You clean the top of the light and apply a blob of silicone in the middle of the solar panel, then put a piece of polythene on top. Using the polythene as a barrier you can squish and spread the silicone into a thick but even layer that seals the top of the light completely. Once the silicone has had some time to start curing (overnight) the polythene film will peel off allowing the curing to complete.
The polythene I used was from clear component bags. It's a very slippery plastic that the silicone does not readily bond to, so it peels off easily. I also noticed that if you apply a blob of silicone onto polythene and then place another piece on top you can create a thick sheet of silicone by using a flat object to press down on it, with some coins as a depth stop on all sides if you need an even thickness.
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