They strip the old finish, then sand and treat the entire surface so it’s ready to accept the new finish. Similar process to using “tough as tile”—only I sanded/cleaned/treated the entire thing by hand and it me took days, they had power tools and were done in a few hours. For mine they used synthetic porcelain resin, there are probably other finish options. Can’t speak to the durability yet as it’s only been done for a week, but it looks brand new and feels higher quality than the tough as tile finish. You cannot use certain harsh cleaning products on the new finish, they gave me cleaning and care instructions. I did not want any of my tile touched, it’s vintage and wraps around the entire room. It would’ve been an entire bathroom renovation. So I went for this option. $774

Edit: it was done by a tub and tile company