I’ve been looking into this a little for our kitchen. Sealants that give a ‘wet look’ prevent the tiles breathing, and if water gets in either from the edges (seeping through cracks in grouting, or from below) then that type develops white patches under the sealant that can never be removed without harming the tile.
A question from me - has anyone used an indoor sealant on clay floor tiles, and if so, which? I would like to protect our recovered kitchen tiles, and probably the livingroom tiles too.