Porcelain tile problem

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Hello, been to see a job, where owner has laid polished black porcelain floor tiles, grouted but apparently no sealer at all, so there's a 'smeary' look to the tiles, owner says he's tried stone cleaner and hefty concentrated floor cleaner, which has brought it up a little but not enough.
I've thought of stone stripper first, then a seal, but anyone used stripper on polished porcelain with good results?
Thanks in advance.....