The instructions for mine (from Victoria plum), stated seal the outside edge only.
Theoretically, it should allow any water that does get past the seals, to drain back into the bath/shower, rather than rotting the channel from the inside!
I did ensure the wall was waterproofed before attaching the screen and used a blob of silicone in the fixing holes before screwing it in.
I also read that outside only, but when I recently removed our screen to properly clean it and regrout, the scuzzy orangey stuff under it was minging... Maybe the screens need a few mm Packers under them when fixing at the sides. Would allow better external sealant application and better internal drainage too...??
It has always struck me that most shower screens are badly designed. After a few years you start to see limescale on the outer face, particularly where the lowest rails meet the uprights. I have often wondered why they don't drill drainage holes like you find on the underside of uPVC windows.
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