For the sealer what about 'Seal Guard'? Just google Seal guard sealant and it'll come up. Very glad I discovered it myself. Comes in a spray or brush on, no need to clean off after, sound and looks very promising to me even though I haven't used it yet myself, but bought a can after much research for a mosaic project soon to be finished. And have you tried simple white vinegar for the limescale?
