Super heated tiles that roll off of the production line are sprayed with water to cool them down before stacking. Sometimes a build up of dust and silica can remain on the underside of the tile; and when it is stacked on top of another tile; can cool to leave a hard to remove residue on the surface of the tile.
This can sometimes go un-noticed in larger factories because the tiles are collected and then packaged at the end of a production line by robots.
CLEANING: LTP POWER STRIPPER 1 & 5 ltr
Leave for 15 minutes. Add a little water and agitate again. Rinse with water and leave to dry.
RESULTS: A GRID FREE CLEAN SURFACE
If a slight residue remains once dry, repeat the above process.
Wherever possible clean the tiles prior to fixing. Cleaning solutions applied to vertical surfaces are not able to stay in place long enough to be as effective, making removal much slower. If cleaning a vertical surface start from the top and work down.