Will these rust if they get damp behind them? Being stainless steel I would think not, but I have seen brass switches rust before after a room was skimmed.
They shouldn't 'rust', but they may discolour, or the lacquer finish may discolour through contact with chemical laden moisture wicked up from the plaster or paint.
Most fitting manufacturers used to recommend allowing plaster and paintwork to fully dry / cure for several weeks before fitting metal accessories, but few installers have time to actually do that.
Brass naturally tarnishes anyway, so it's protected by a coat of clear lacquer. Trouble is, that same lacquer makes it impossible to clean them effectively when the brass does tarnish.
The last metal switch I fitted came with a clear plastic 'gasket' to sandwich between the switch plate and wall to stop the metal coming into direct contact with the plaster.
If they truly are stainless then they should be ok, but stainless steel is an expensive metal, so the majority of 'stainless' accesories are not actually made from stainless.
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